EARTH
, Terra, in Natural Philosophy, one of the four vulgar, or Peripatetical elements; defined a simple, dry, and cold substance; and, as such, an ingredient in the composition of all natural bodies.
The Earth, in Geography, this terraqueous globe or ball, which we inhabit, consisting of land and sea.
Figure of the Earth. The ancients had various opinions as to the figure of the earth: some, as Anaximander and Leucippus, held it cylindrical, or in form of a drum: but the principal opinion was, that it was flat; that the visible horizon was the bounds of the earth, and the ocean the bounds of the horizon; that the heavens and earth above this ocean were the whole visible universe; and that all beneath the ocean was Hades: and of this same opinion were also some of the Christian fathers, as Lactantius, St. Augustine, &c. See Lactan. lib. 3, cap. 24; St. Aug. lib. 16, de Civitate Dei; Aristotle de Cœlo, lib. 2, cap. 13.
Such of the ancients however as understood any thing of astronomy, and especially the doctrine of eclipses, must have been acquainted with the round figure of the earth; as the ancient Babylonian astronomers, who had calculated eclipses long before the time of Alexander, and Thales the Grecian, who predicted an eclipse of the sun. It is now indeed agreed on all hands, unless perhaps by the most vulgar and ignorant, that the form of the terraqueous globe is globular, or very nearly so.
That the exterior of the earth is round, or rotund, is manifest to the most common perception, in the case of a ship sailing either from the land, or towards it; for when a person stands upon the shore, and sees a ship sail from the land, out to sea; at first he loses sight of the hull and lower parts of the ship, next the rigging and middle parts, and lastly of the tops of the masts themselves, in every case the rotundity of the sea between the ship and the eye being very visible: the contrary happens when a ship sails towards us; we first see the tops of the masts appear just over the rotundity of the sea; next we perceive the rigging, and lastly the hull of the ship itself: all which is well illustrated by the following figure.
The round figure of the earth is also evident from the eclipses of the sun and moon; for in all eclipses of the moon, which are caused by the moon passing through the earth's shadow, that shadow always appears circular upon the face of the moon, what way soever it be projected, whether east, west, north, or south, and howsoever its diameter vary, according to the greater or less distance from the earth. Hence it follows, that the shadow of the earth, in all situations, is really conical; and consequently the body that projects it, i. e. the earth, is at least nearly spherical.
The spherical figure of the earth is also evinced from the rising and setting of the sun, moon, and stars; all which happen sooner to those who live to the east, and later to those living westwardly; and that more or less so, according to the distance and roundness of the earth.
So also, going or sailing to the northward, the north pole and northern stars become more elevated, and the south pole and southern stars more depressed; the elevation northerly increasing equally with the depression southerly; and either of them proportionably to the distance gone. The same thing happens in going to the southward. Besides, the oblique ascensions, de- | scensions, emersions, and amplitudes of the rising and setting of the sun and stars in every latitude, are agreeable to the supposition of the earth's being of a spherical form: all which could not happen if it was of any other figure.
Moreover, the roundness of the earth is farther confirmed by its having been often sailed round: the first time was in the year 1519, when Ferd. Magellan made the tour of the whole globe in 1124 days. In the year 1557 Francis Drake performed the same in 1056 days: in the year 1586, Sir Tho. Cavendish made the same voyage in 777 days; Simon Cordes, of Rotterdam, in the year 1590; in the year 1598, Oliver Noort, a Hollander, in 1077 days; Van Schouten, in the year 1615, in 749 days; Jac. Heremites and Joh. Huygens, in the year 1623, in 802 days: and many others have since performed the same navigation, particularly Anson, Bougainville, and Cook. Sometimes failing round by the eastward, sometimes to the westward; till at length they arrived again in Europe, from whence they set out; and in the course of their voyage, observed that all the phenomena, both of the heavens and earth, correspond to, and evince this spherical figure.
The same globular figure is likewise inferred from the operation of Levelling, in which it is found necessary to make an allowance for the difference between the apparent and the true level.
The natural cause of this sphericity of the globe is, according to Sir Isaac Newton, the great principle of attraction, which the Creator has stamped on all the matter in the universe; and by which all bodies, and all the parts of bodies, mutually attract one another.— And the same is the cause of the sphericity of the drops of rain, quicksilver, &c.
What the earth loses of its sphericity by mountains and valleys, is nothing considerable; the highest eminence being scarce equivalent to the minutest protuberance on the surface of an orange. Its difference from a perfect sphere however is more considerable in another respect, by which it approaches nearly to the shape of an orange, or to an oblate spheroid, being a little flatted at the poles, and raised about the equatorial parts, so that the axis from pole to pole is less than the equatorial diameter. What gave the first occasion to the discovery of this figure of the earth, was the observations of some French and English philosophers in the East-Indies, and other parts, who found that pendulums, the nearer they came to the equator, performed their vibrations slower: from whence it follows, that the velocity of the descent of bodies by gravity, is less in countries nearer to the equator; and consequently that those parts are farther removed from the centre of the earth, or from the common centre of gravity. See the History of the Royal Academy of Sciences, by Du Hamel, p. 110, 156, 206; and l'Hist. de l'Acad. Roy. 1700 and 1701.—This circumstance put Huygens and Newton upon sinding out the cause, which they attributed to the revolution of the earth about its axis. If the earth were in a fluid state, its rotation round its axis would necessarily make it put on such a figure, because the centrifugal force being greatest towards the equator, the fluid would there rise and swell most; and that its figure really should be so now, seems necessary, to keep the sea in the equinoctial re- gions from overflowing the earth about those parts. See this curious subject well handled by Huygens, in his discourse De Causa Gravitatis, pa. 154, where he states the ratio of the polar diameter to that of the equator, as 577 to 578. And Newton, in his Principia, first published in 1686, demonstrates, from the theory of gravity, that the figure of the earth must be that of an oblate spheroid generated by the rotation of an ellipse about its shortest diameter, provided all the parts of the earth were of an uniform density throughout, and that the proportion of the polar to the equatorial diameter of the earth, would be that of 689 to 692, or nearly that of 229 to 230, or as .9956522 to 1.
This proportion of the two diameters was calculated by Newton in the following manner. Having found that the centrifugal force at the equator is 1/289th of gravity, he assumes, as an hypothesis, that the axis of the earth is to the diameter of the equator as 100 to 101, and thence determines what must be the centrifugal force at the equator to give the earth such a form, and finds it to be 4/505ths of gravity: then, by the rule of proportion, if a centrifugal force equal to 4/805ths of gravity would make the earth higher at the equator than at the poles by 1/100th of the whole height at the poles, a centrifugal force that is the 1/289th of gravity will make it higher by a proportional excess, which by calculation is 1/229th of the height at the poles; and thus he discovered that the diameter at the equator is to the diameter at the poles, or the axis, as 230 to 229. But this computation supposes the earth to be every where of an uniform density; whereas if the earth is more dense near the centre, then bodies at the poles will be more attracted by this additional matter being nearer; and therefore the excess of the semi-diameter of the equator above the semi-axis, will be different. According to this proportion between the two diameters, Newton farther computes, from the different measures of a degree, that the equatorial diameter will exceed the polar, by 34 miles and 1/5.
Nevertheless, Messrs Cassini, both father and son, the one in 1701, and the other in 1713, attempted to prove, in the Memoirs of the Royal Academy of Sciences, that the earth was an oblong spheroid; and in 1718, M. Cassini again undertook, from observations, to shew that, on the contrary, the longest diameter passes through the poles; which gave occasion for Mr. John Bernoulli, in his Essai d'une Nouvelle Physique Celeste, printed at Paris in 1735, to triumph over the British philosopher, apprehending that these observations would invalidate what Newton had demonstrated. And in 1720, M. De Mairan advanced arguments, supposed to be strengthened by geometrical demonstrations, farther to confirm the assertions of Cassini. But in 1735 two companies of mathematicians were employed, one for a northern, and another for a southern expedition, the result of whose observations and measurement plainly proved that the earth was flatted at the poles. See Degree.
The proportion of the equatorial diameter to the polar, as stated by the gentlemen employed on the northern expedition for measuring a degree of the meridian, is as 1 to 0.9891; by the Spanish mathematicians as 266 to 265, or as 1 to 0.99624; by M. Bouguer as 179 to 178, or as 1 to 0.99441.
As to all conclusions however deduced from the | length of pendulums in different places, it is to be observed that they proceed upon the supposition of the uniform density of the earth, which is a very improbable circumstance; as justly observed by Dr. Horsley in his letter to Capt. Phipps. “You sinish your article, he concludes, relating to the pendulum with saying, ‘that these observations give a figure of the earth nearer to Sir Isaac Newton's computation, than any others that have hitherto been made;’ and then you state the several figures given, as you imagine, by former observations, and by your own. Now it is very true, that if the meridians be ellipses, or if the figure of the earth be that of a spheroid generated by the revolution of an ellipsis, turning on its shorter axis, the particular figure, or the ellipticity of the generating ellipsis, which your observations give, is nearer to what Sir Isaac Newton saith it should be, if the globe were homogeneous, than any that can be derived from former observations. But yet it is not what you imagine. Taking the gain of the pendulum in latitude 79° 50′ exactly as you state it, the difference between the equatorial and the polar diameter, is about as much less than the Newtonian computation makes it, and the hypothesis of homogeneity would require, as you reckon it to be greater. The proportion of 212 to 211 should indeed, according to your observations, be the proportion of the force that acts upon the pendulum at the poles, to the force acting upon it at the equator. But this is by no means the same with the proportion of the equatorial diameter to the polar. If the globe were homogeneous, the equatorial diameter would exceed the polar by 1/230 of the length of the latter: and the polar force would also exceed the equatorial by the like part. But if the difference between the polar and equatorial force be greater than 1/230, (which may be the case in an heterogeneous globe, and seems to be the case in ours,) then the difference of the diameters should, according to theory, be less than 1/230, and vice versa.
“I confess this is by no means obvious at first sight; so far otherwise, that the mistake, which you have fallen into, was once very general. Many of the best mathematicians were misled by too implicit a reliance upon the authority of Newton, who had certainly consined his investigations to the homogeneous spheroid, and had thought about the heterogeneous only in a loofe and general way. The late Mr. Clairault was the first who set the matter right, in his elegant and subtle treatise on the figure of the earth. That work hath now been many years in the hands of mathematicians, among whom I imagine there are none, who have considered the subject attentively, that do not acquiesce in the author's conclusions.
“In the 2d part of that treatise, it is proved, that putting *r for the polar force, *p for the equatorial, d for the true ellipticity of the earth's figure, and e for the ellipticity of the homogeneous spheriod, : therefore ; and therefore, according to your observation, d=1/251. This is the just conclusion from your observations of the pendulum, taking it for granted, that the meridians are ellipses: which is an hypothesis, upon which all the reasonings of theory have hitherto proceeded. But plausible as it may seem, I must say, that there is much reason from experiment to call it in question. If it were true, the increment of the force which actuates the pendulum, as we approach the poles, should be as the square of the sine of the latitude: or, which is the same thing, the decrement, as we approach the equator, should be as the square of the cosine of the latitude. But whoever takes the pains to compare together such of the observations of the pendulum in different latitudes, as seem to have been made with the greatest care, will find that the increments and decrements do by no means follow these proportions; and in those which I have examined, I sind a regularity in the deviation which little resembles the mere error of observation. The unavoidable conclusion is, that the true figure of the meridians is not elliptical. If the meridians are not ellipses, the difference of the diameters may indeed, or it may not, be proportional to the difference between the polar and the equatorial force; but it is quite an uncertainty, what relation subsists between the one quantity and the other; our whole theory, except so far as it relates to the homogeneous spheroid, is built upon false assumptions, and there is no saying what figure of the earth any observations of the pendulum give.”
He then lays down the following table, which shews the different results of observations made in different latitudes; in which the first three columns contain the names of the several observers, the places of observation, and the latitude of each; the 4th column shews the quantity of *r-*p in such parts as *p is 100000, as deduced from comparing the length of the pendulum at each place of observation, with the length of the equatorial pendulum as determined by M. Bouguer, upon the supposition that the increments and decrements of force, as the latitude is increased or lowered, observe the proportion which theory assigns. Only the 2d and the last value of *r-*p are concluded from comparisons with the pendulum at Greenwich and at London, not at the equator. The 5th column shews the value of d corresponding to every value of *r-*p, according to Clairault's theorem:
| Observers. | Places. | Latitudes. | *r-*p | d | |||
| Bouguer | Equator | 0° | 0′ | ||||
| Bouguer | Porto Bello | 9 | 34 | 741.8 | 1/784 | ||
| Green | Otaheitee | 17 | 29 | 563.2 | 1/326 | ||
| Bouguer | San Domingo | 18 | 27 | 591.0 | 1/368 | ||
| Abbé de la Caille | } | Cape of Good Hope | } | 33 | 55 | 731.5 | |
| " | Paris | 48 | 50 | 585.1 | 1/361 | ||
| The Academicians | } | Pello | 66 | 48 | 565.9 | 1/329 | |
| Capt. Phipps | " | 79 | 50 | 471.2 | 1/2<*>1 |
“By this table it appears, that the observations in the middle parts of the globe, setting aside the single one at the Cape, are as consistent as could reasonably be expected; and they represent the ellipticity of the earth as about 1/340. But when we come within 10 degrees of the equator, it should seem that the force of gravity suddenly becomes much less, and within the like | distance of the poles much greater than it could be in such a spheroid.”
The following problem, communicated by Dr Leatherland to Dr. Pemberton, and published by Mr. Robertson, serves for finding the proportion between the axis and the equatorial diameter, from measures taken of a degree of the meridian in two different latitudes, supposing the earth an oblate spheroid.
Let APap be an ellipse representing a section of the earth through the axis Pp; the equatorial diameter, or the greater axis of the ellipse, being Aa; let E and F be two places where the measure of a degree has been taken; these measures are proportional to the radii of curvature in the ellipse at those places; and if CQ, CR be conjugates to the diameters whose vertices are E and F, CQ will be to CR in the subtriplicate ratio of the radius of curvature at E to that at F, by Cor. 1, prop. 4, part 6 of Milnes's Conic Sections, and therefore in a given ratio to one another; also the angles QCP, RCP are the latitudes of E and F; so that, drawing QV parallel to Pp, and QXYW to Aa, these angles being given, as well as the ratio of CQ to CR, the rectilinear figure CVQXRY is given in species; and the ratio of to is given, which is the ratio of CA2 to CP2; therefore the ratio of CA to CP is given.
Hence, if the sine and cosine of the greater latitude be each augmented in the subtriplicate ratio of the measure of the degree in the greater latitude to that in the lesser, then the difference of the squares of the augmented sine, and the sine of the lesser latitude, will be to the difference of the squares of the cosine of the lesser latitude and the augmented cosine, in the duplicate ratio of the equatorial to the polar diameter. For, Cq being taken in CQ equal to CR, and qv drawn parallel to QV, Cv and vq, CZ and ZR will be the signs and cosines of the respective latitudes to the same radius; and CV, VQ will be the augmentations of Cv and Cq in the ratio named.
Hence, to find the ratio between the two axes of the earth, let E denote the greater, and F the lesser of the two latitudes, M and N the respective measures taken in each; and let P denote √3M/N: then .
It also appears by the above problem, that when one of the degrees measured, is at the equator, the cosine of the latitude of the other being augmented in the subtriplicate ratio of the degrees, the tangent of the latitude will be to the tangent answering to the augmented cosine, in the ratio of the greater axis to the less. For supposing E the place out of the equator; then if the semi-circle Plmnp be described, and lC joined, and mo drawn parallel to aC: Co is the cosine of the latitude to the radius CP, and CY that cosine augmented in the ratio before-named; YQ being to Yl, that is Ca to Cn or CP, as the tangent of the angle YCQ, the latitude of the point E, to the tangent of the angle YCl, belonging to the augmented cosine. Thus, if M represent the measure in a latitude denoted by E, and N the measure at the equator, let A denote an angle whose measure is Then .
But M, or the length of a degree, obtained by actual mensuration in different latitudes, is known from the following table:
| Names. | Latit. | Value of M. | ||||
| ° | ′ | toises | ||||
| Maupertuis and Assoc. | 66 | 20 | M = 57438 | |||
| Cassini and La Caille | { | 49 | 22 | M = 57074 | ||
| 45 | 00 | M = 57050 | ||||
| Boscovich | 43 | 00 | M = 56972 | |||
| De la Caille | 33 | 18 | M = 57037 | |||
| Juan and Ulloa | 00 | 00 | M = 56768 | } | at the equator. | |
| Bouguer | 00 | 00 | M = 56753 | |||
| Condamine | 00 | 00 | M = 56749 |
Now, by comparing the 1st with each of the following ones; the 2d with each of the following; and in like manner the 3d, 4th, and 5th, with each of the following; there will be obtained 25 results, each shewing the relation of the axes or diameters; the arithmetical means of all of which will give that ratio as 1 to 0.9951989.
If the measures of the latitudes of 49° 22′, and of 45°, which fall within the meridian line drawn through France, and which have been re-examined and corrected since the northern and southern expedition, be compared with those of Maupertuis and his associates in the north, and that of Bouguer at the equator, there will result 6 different values of the ratio of the two axes, the arithmetical mean of all which is that of 1 to 0.9953467, which may be considered as the ratio of the greater axis to the less; which is as 230 to 228.92974, or 215 to 214, or very near the ratio as assigned by Newton.
Now, the magnitude as well as the sigure of the earth, that is the polar and equatorial diameters, may be deduced from the foregoing problem. For, as half the latus rectum of the greater axis Aa is the radius of curvature at A, it is given in magnitude from the degree measured there, and thence the axes themselves are given. Thus, the circular are whose length is equal to the radius being 57.29578 degrees, if this number be multiplied by 56750 toises, the measure of a degree at | the equator, as Bouguer has stated it, the product will be the radius of curvature there, or half the latus rectum of the greater axis; and this is to half the lesser axis in the ratio of the less axis to the greater, that is, as 0.9953467 to 1; whence the two axes are 6533820 and 6564366 toises, or 7913 and 7950 English miles; and the difference between the two axes about 37 miles. See Robertson's Navigation, vol. 2, pa. 206 &c. See also Suite des Mem. de l'Acad. 1718, pa. 247, and Maclaurin's Fluxions, vol. 2, book 1, ch. 14.
And very nearly the same ratio is deduced from the lengths of pendulums vibrating in the same time, in different latitudes; provided it be again allowed that the meridians are real ellipses, or the earth a true spheroid, which however can only take place in the case of an uniform gravity in all parts of the earth.
Thus, in the new Petersburgh Acts, for the years 1788 and 1789, are accounts and calculations of experiments relative to this subject, by M. Krafft. These experiments were made at different times and in various parts of the Russian empire. This gentleman has collected and compared them, and drawn the proper conclusions from them: thus he infers that the length x of a pendulum that swings seconds in any given latitude l, and in a temperature of 10 degrees of Reaumur's thermometer, may be determined by this equation:
sine 2l, lines of a French foot, or sine 2l, in English inches, in the temperature of 53 of Fahrenheit's thermometer.
This expression nearly agrees, not only with all the experiments made on the pendulum in Russia, but also with those of Mr. Graham in England, and those of Mr. Lyons in 79° 50′ north latitude, where he found its length to be 431.38 lines. It also shews the augmentation of gravity from the equator to the parallel of a given latitude l: for, putting g for the gravity under the equator, G for that under the pole, and y for that under the latitude l, M. Krafft finds ; and theref. G= 1.0052848 g.
From this proportion of gravity under different latitudes, the same author infers that, in case the earth is a homogeneous ellipsoid, its oblateness must be 1/191; instead of 1/2<*>0, which ought to be the result of this hypothesis: but on the supposition that the earth is a heterogeneous ellipsoid, he finds its oblateness, as deduced from these experiments, to be 1/297; which agrees with that resulting from the measurement of some of the degrees of the meridian. This confirms an observation of M. De la Place, that, if the hypothesis of the earth's homogeneity be given up, then theory, the measurement of degrees of latitude, and experiments with the pendulum, all agree in their result with respect to the oblateness of the earth. See Memoires de l' Acad. 1783, pa. 17.
In the Philos. Trans. for 1791, pa. 236, Mr. Dalby has given some calculations on measured degrees of the meridian, from whence he infers, that those degrees measured in middle latitudes, will answer nearly to an ellipsoid whose axes are in the ratio assigned by Newton, viz, that of 230 to 229. And as to the deviations of some of the others, viz, towards the poles and equator, he thinks they are caused by the errors in the observed celestial arcs.
Tacquet draws some pretty little inferences, in the form of paradoxes, from the round figure of the earth; as, 1st, That if any part of the surface of the earth were quite plane, a man could no more walk upright upon it, than on the side of a mountain. 2d, That the traveller's head goes a greater space than his feet; and a horseman than a footman; as moving in a greater circle. 3d, That a vessel, full of water, being raised perpendicularly, some of the water will be continually flowing out, yet the vessel still remain full; and on the contrary, if a vessel of water be let perpendicularly down, though nothing flow out, yet it will cease to be full: consequently there is more water contained in the same vessel at the foot of a mountain, than on the top; because the surface of the water is compressed into a segment of a smaller sphere below than above. Tacq. Astron. lib. 1, cap. 2.
Changes of the Earth. Mr. Boyle suspects that there are great, though slow internal changes, in the mass of the earth. He argues from the varieties observed in the change of the magnetic needle, and from the observed changes in the temperature of climates. But as to the latter, there is reason to doubt that he could not have diaries of the weather sufficient to direct his judgment. Boyle's Works abr. vol. 1, pa. 292, &c.
Magnetism of the Earth. The notion of the magnetism of the earth was started by Gilbert; and Boyle supposes magnetic effluvia moving from one pole to the other. See his Works abr. vol. 1, pa. 285, 290.
Dr. Knight also thinks that the earth may be considered as a great loadstone, whose magnetical parts are disposed in a very irregular manner; and that the south pole of the earth is analogous to the north pole in magnets, that is, the pole by which the magnetical stream enters. See Magnet.
He observes, that all the phenomena attending the direction of the needle, in different parts of the earth, in great measure correspond with what happens to a needle, when placed upon a large terrella; if we make allowances for the different dispositions of the magnetical parts, with respect to each other, and consider the south pole of the earth as a north pole with regard to magnetism. The earth might become magnetical by the iron ores it contains, for all iron ores are capable of magnetism. It is true, the globe might notwithstanding have remained unmagnetical, unless some cause had existed capable of making that repellent matter producing magnetism move in a stream through the earth.
Now the doctor thinks that such a cause does exist. For if the earth revolves round the sun in an ellipsis, and the south pole of the earth is directed towards the sun, at the time of its descent towards it, a stream of repellent matter will thence be made to enter at the south pole, and issue out at the north. And he suggests, that the earth's being in its perihelion in winter may be one reason why magnetism is stronger in this season than in summer.
This cause here assigned for the earth's magnetism must continue, and perhaps improve it, from year to year. Hence the doctor thinks it probable, that the earth's magnetism has been improving ever since the creation, and that this may be one reason why the use | of the compass was not discovered sooner. See Dr. Knight's Attempt to demonstrate, that all the phenomena in nature may be explained by Attraction and Repulsion, prop. 87.
Magnitude and Constitution of the Earth. This has been variously determined by different authors, both ancient and modern. The usual way has been, to measure the length of one degree of the meridian, and multiply it by 360, for the whole circumference. See Degree. Diogenes Laertius informs us, that Anaximander, a scholar of Thales, who lived about 550 years before the birth of Christ, was the first who gave an account of the circumference of the sea and land; and it seems his measure was used by the succeeding mathematicians, till the time of Eratosthenes. Aristotle, at the end of lib. 2 De Cœlo, says, the mathematicians who have attempted to measure the circuit of the earth, make it 40000 stadiums: which it is thought is the number determined by Anaximander.
Eratosthenes, who lived about 200 years before Christ, was the next who undertook this business; which, as Cleomedes relates, he performed by taking the sun's zenith distances, and measuring the distance between two places under the same meridian; by which he deduced for the whole circuit about 250000 stadiums, which Pliny states at 31500 Roman miles, reckoning each at 1000 paces. But this measure was accounted false by many of the ancient mathematicians, and particularly by Hipparchus, who lived 100 years afterwards, and who added 25000 stadiums to the circuit of Eratosthenes.
Possidonius, in the time of Cicero and Pompey the Great, next measured the earth, viz, by means of the altitudes of a star, and measuring a part of a meridian; and he concluded the circumference at 240000 stadiums, according to Cleomedes, but only at 180000 according to Strabo.
Ptolomy, in his Geography, says that Marinus, a celebrated geographer, attempted something of the same kind; and, in lib. 1, cap. 3, he mentions that he himself had tried to perform the business in a way different from any other before him, which was by means of places under different meridians: but he does not say how much he made the number; for he still made use of the 180000, which had been found out before him.
Snellius relates, from the Arabian Geographer Abelfedea, who lived about the 1300th year of Christ, that about the 800th year of Christ, Almaimon, an Arabian king, having collected together some skilful mathematicians, commanded them to find out the circumference of the earth. Accordingly these made choice of the fields of Mesopotamia, where they measured under the same meridian from north to south, till the pole was depressed one degree lower: which measure they found equal to 56 miles, or 56 1/2: so that according to them the circuit of the earth is 20160 or 20340 miles.
It was a long time after this before any more attempts were made in this business. At length however, the same Snell, above mentioned, professor of mathematies at Leyden, about the year 1620, with great skill and labour, by measuring large distances between two parallels, found one degree equal to 28500 perches, each of which is 12 Rhinland feet, amounting to 19 Dutch miles, and so the whole periphery 6840 miles; a mile being, according to him, 1500 perches, or 18000 Rhinland feet. See his treatise called Eratosthenes Batavus.
The next that undertook this measurement, was Richard Norwood, who in the year 1635, by measuring the distance from London to York with a chain, and taking the sun's meridian altitude, June 11th old style, with a sextant of about 5 feet radius, found a degree contained 367200 feet, or 69 miles and a half and 14 poles; and thence the circumference of a great circle of the earth is a little more than 25036 miles, and the diameter a little more than 7966 miles. See the particulars of this measurement in his Seaman's Practice.
The measurement of the earth by Snell, though very ingenious and troublesome, and much more accurate than any of the ancients, being still thought by some French mathematicians, as liable to certain small errors, the business was renewed, after Snell's manner, by Picard and other mathematicians, by the king's command; using a quadrant of 3 1/6 French feet radius; by which they found a degree contained 342360 French feet. See Picard's treatise, La Mesure de la Terre.
M. Cassini the younger, in the year 1700, by the king's command also, renewed the business with a quadrant of 10 feet radius, for taking the latitude, and another of 3 1/8 feet for taking the angles of the triangles; and found a degree, from his calculation, contained 57292 toises, or almost 69 1/2 English miles.
See the results of many other measurements under the article Degree. From the mean of all which, the following dimensions may be taken as near the truth:
| the circumference | 25000 miles, |
| the diameter | 7957 3/4 miles, |
| the superficies | 198944206 square miles, |
| the solidity | 263930000000 cubic miles. |
It is now generally granted that the terraqueous globe has two motions, besides that on which the precession of the equinoxes depends; the one diurnal around its own axis in the space of 24 hours, which constitutes the natural day or nycthemeron; the other annual, about the sun, in an elliptical orbit or track, in 365 days 6 hours, constituting the year. From the former arise the diversities of night and day; and from the latter, the vicissitudes of seasons, spring, summer, autumn, winter.
The terraqueous globe is distinguished into three parts or regions, viz, 1st, The external part or crust, being that from which vegetables spring and animals are nursed. 2d, The middle, or intermediate part, which is possessed by fossils, extending farther than human labour ever yet penetrated. 3d, The internal or central part, which is utterly unknown to us, though by many authors supposed of a magnetic nature; by others, a mass or sphere of fire; by others, an abyss or collection of waters, surrounded by the strata of earth; and by others, a hollow, empty space, inhabited by animals, who have their sun, moon, planets, and | other conveniences within the same. But others divide the body of the globe into two parts, viz, the external part, called the cortex, including the internal, which they call the nucleus, being of a different nature from the former, and possessed by fire, water, or more probably by a considerable portion of metals, as it has been found, by calculation, that the mean density of the whole earth is near double the density of common stone. See my determination of it, Philos. Trans. 1778, pa. 781.
The external part of the globe either exhibits inequalities, as mountains and valleys; or it is plane and level; or dug in channels, fissures, beds, &c, for rivers, lakes, seas, &c. These inequalities in the face of the earth most naturalists suppose have arisen from a rupture or subversion of the earth, by the force either of the subterraneous sires or waters. The earth, in its natural and original state, it has been supposed by Des Cartes, and after him, Burnet, Steno, Woodward, Whiston, and others, was perfectly round, smooth, and equable; and they account for its present rude and irregular form, principally from the great deluge. See Deluge.
In the external, or cortical part of the earth, there appear various strata, supposed the sediments of several sloods; the waters of which, being replete with matters of divers kinds, as they dried up, or oozed through, deposited these different matters, which in time hardened into strata of stone, sand, coal, clay, &c.
Dr. Woodward has considered the circumstances of these strata with great attention, viz, their order, number, situation with respect to the horizon, depth, intersections, fissures, colour, consistence, &c. He ascribes the origin and formation of them all, to the great flood or cataclysmus. At that terrible revolution he supposed that all sorts of terrestrial bodies had been dissolved and mixed with the waters, forming all together a chaos or confused mass. This mass of terrestrial particles, intermixed with water, he supposes was at length precipitated to the bottom; and that generally according to the order of gravity, the heaviest sinking first, and the lightest afterwards. By such means were the strata formed of which the earth consists; which, attaining their solidity and hardness by degrees, have continued so ever since. These sediments, he farther concludes, were at first all parallel and concentrical; and the surface of the earth formed of them, perfectly smooth and regular; but that in course of time, divers changes happening, from earthquakes, volcanos, &c, the order and regularity of the strata was disturbed and broken, and the surface of the earth by such means brought to the irregular form in which it now appears.
M. De Buffon surmises that the earth, as well as the other planets, are parts struck off from the body of the sun by the collision of comets; and that when the earth assumed its form, it was in a state of liquefaction by fire. But that could not be the method of producing the planets; for if they were struck off from the body of the sun, they would move in orbits that would always pass through the sun, instead of having the sun for their socus, or centre, as they are now found; so that having been struck off they would fall down into the sun again, terminating their career as it were after one revolution only.
Earth, in Astronomy, is one of the primary planets, according to the system of Copernicus, or Pythagoras; its astronomical character or mark being : but according to the Ptolomaic hypothesis, the earth is the centre of the system. For, whether the earth move or remain at rest, that is, whether it be fixed in the centre, having the sun, the heavens and stars moving round it from east to west; or, these being at rest, whether the earth only moves from west to east, is the great article that distinguishes the Ptolomaic system from the Copernican.
Motion of the Earth. It is now universally agreed that, besides the small motion of the earth which causes the precession of the equinoxes, the earth has two great and independent motions; viz, the one by which it turns round its own axis, in the space of 24 hours nearly, and causing the continual succession of day and night; and the other an absolute motion of its whole mass in a large orbit about the sun, having that luminary for its centre, in such manner that its axis keeps always parallel to itself, inclined in the same angle to its path, and by that means causing the vicissitudes of seasons, spring, summer, autumn, winter.
It is indeed true that, as to sense, the earth appears to be fixed in the centre, with the sun, stars and heavens moving round it every day; and such doubtless would be considered as the true nature of the motions in the rude ages of mankind, as they are still by the rude and unlearned. But to a thinking and learned mind, the contrary will soon appear.
Indeed there are traces of the knowledge of these motions in the earliest age of the sciences. Cicero, in his Tusc. Quæst. says that Nicetas of Syracuse first discovered that the earth had a diurnal motion, by which it revolved round its axis every 24 hours; and Plutarch, de Placit. Philosoph. informs, that Philolaus discovered its annual motion round the sun; and Aristarchus, about 100 years after Philolaus, proposed the motion of the earth in stronger and clearer terms, as we are assured by Archimedes, in his Arenarius. And the same, we are farther assured, was the opinion and doctrine of Pythagoras.
But the religious opinions of the heathen world prevented this doctrine from being more cultivated. For, Aristarchus being accused of sacrilege by Cleanthes for moving Vesta and the tutelar deities of the universe out of their places, the philosophers were obliged to dissemble, and seem to relinquish so perilous a position.
Many ages afterwards, Nic. Cusanus revived the ancient system, in his Doct. de Pignorant. and asserted the motion of the earth: but the doctrine gained very little ground till the time of Copernicus, who shewed its great use and advantages in astronomy; and who had immediately all the philosophers and astronomers on his side, who dared to differ from the crowd, and were not afraid of ecclesiastical censure, which was not less dangerous under the christian dispensation, than it had been under that of the heathen. For, because certain parts of scripture make mention of the stability | of the earth, and of the motion of the sun, as the rising and setting, &c, the fathers of the church thought their religion required that they should defend, with all its power, what they conceived to be its doctrines, and to censure and punish every attempt at innovation on such points. They have now however been pretty generally convinced that in such instances the expressions are only to be considered as accommodated to appearances, and the vulgar notions of things.
By the diurnal rotation of the earth on its axis, the same phenomena will take place as if it had no such motion, and as if the sun and stars moved round it. For, turning round from west to east, causes the sun and all the visible heavens to seem to move the contrary way, or from east to west, as we daily see them do. So, when in its rotation it has brought the sun or a star to appear just in the horizon in the east, they are then said to be rising; and as the earth continues to revolve more and more towards the east, other stars seem to rise and advance westwards, passing the meridian of the observer, when they are due south from him, and at their greatest altitude above his horizon; after which, by a continuance of the same motions, viz, of the earth's rotation eastwards, and the luminaries apparent counter motion westwards, these decline from the meridian, or south point, towards the west, where being arrived, they are said to set and descend below it; and so on continually from day to day; thus making it day while the sun is above the horizon, and night while he is below it.
While the earth is thus turning on its axis, it is at the same time carried by its proper motion in its orbit round the sun, as one of the planets, namely, between the orbits of Venus and Mars, having the orbits of Venus and Mercury within its own, or between it and the sun, in the centre, and those of Mars, Jupiter, Saturn, &c, without or above it; which are therefore called superior planets, and the others the inferior ones. This is called the annual motion of the earth, because it is performed in a year, or 365 days 6 hours nearly; or rather 365 days 5h 49m, from any equinox or solstice to the same again, making the tropical year; but from any fixed star to the same again, as seen from the sun, in 365 days 6 hours 9 minutes, which is called the sidereal year. The figure of this orbit is elliptical, having the sun in one focus, the mean distance being about 95 millions of miles, which is upon the supposition that the sun's parallax is about 8″ <*>, or the angle under which the earth's semi-diameter would appear to an observer placed in the sun: and the eccentricity of the orbit, or distance of the sun, in the focus, from the centre of this elliptic orbit, is about 1/60th of the mean distance.
Now this annual motion is performed in such a manner, that the earth's axis is every where parallel, or in the same direction in every part of the orbit; by which means it happens, that at one time of the year the sun enlightens more of the north polar parts, and at the opposite season of the year more of the southern parts, thus shewing all the varieties of seasons, spring, summer, autumn and winter; which may be illustrated in the following manner: Let the candle I (fig. 1, plate viii) represent the sun, about which the earth E, or F, &c, is moved in its elliptical orbit ABCD, or ecliptic, and cutting the equator abcd in the nodes E and G: then, suspending the terrella by its north pole, and moving it, so suspended, round the ecliptic, its axis will always be parallel to its first position, and the various seasons will be represented at the different parts of the path. Thus, when the earth is at or F, the enlightened half of it includes the south pole, and leaves the north pole in darkness, making our winter; at G it is spring, and the two poles are equally illuminated, and the days are every where of the same length; at H or it is our summer, the north polar parts being in the illuminated hemisphere, and the southern in the dark one; lastly at E it is autumn, the poles being equally illuminated again, and the days of equal length every where.
Earth, its Quantity of Matter, Density, and Attractive Power. Although the relative densities of the earth and most of the other planets have been known a considerable time, it is but very lately that we have come to the knowledge of the absolute gravity or density of the whole mass of the earth. This I have calculated and deduced from the observations made by Dr. Maskelyne, Astronomer Royal, at the mountain Schehallien, in the years 1774, 5, and 6. The attraction of that mountain on a plummet, being observed on both sides of it, and its mass being computed from a number of sections in all directions, and consisting of stone; these data being then compared with the known attraction and magnitude of the earth, gave by proportion its mean density, which is to that of water as 9 to 2, and to common stone as 9 to 5: from which very considerable mean density, it may be presumed that the internal parts contain some great quantities of metals.
From the density, now found, its quantity of matter becomes known, being equal to the product of its density by its magnitude. From various experiments too, we know that its attractive force, at the surface, is such, that bodies fall there through a space of 16 1/12 feet in the first second of time: from whence the force at any other place, either within or without it, becomes known; for the force at any part within it, is directly as its distance from the centre; but the force of any part without it, reciprocally as the square of its distance from the centre.
