, the opposite to cold, being a relative term denoting the property of fire, or of those bodies we denominate hot; being in us a sensation excited by the action of fire.

Heat, as it exists in the hot body, or that which constitutes and denominates a body hot, and enables it to produce such effects on our organs, is variously considered by the philosophers: some making it a quality, others a substance, and others only a mechanical affection. The former principle is laid down by Aristotle and the Peripatetics. While the Epicureans, and other corpuscularians, define Heat not as an accident of fire, but as an essential power or property of it, the same in reality with it, and only distinguished from it in the manner of our conception. So that Heat, on their principles, is no other than the volatile substance of fire itself, reduced into atoms, and emitted in a continual stream from ignited bodies; so as not only to warm the objects within its reach, but also, if they be inflammable, to kindle them, turn them into fire, and conspire with them to make flame. In effect, these corpuscles, say they, flying off from the ignited body, constitute fire while yet contained within the sphere of its flame; but when fled, or got beyond the same, and dispersed every way, so as to escape the apprehension of the eye, and only to be perceived by the feeling, they take the denomination of Heat, inasmuch as they excite in us that sensation.

The Cartesians, improving on this doctrine, assert that Heat consists in a certain motion of the insensible particles of a body, resembling the motion by which the several parts of our body are agitated by the motion of the heart and blood.

Our latest and best writers of mechanical, experimental, and chemical philosophy, differ very considerably about Heat. The chief difference is, whether it be a peculiar property of one certain immutable body, called fire, or phlogiston, or electricity; or whether it may be produced mechanically in other bodies, by inducing an alteration in their particles. The former tenet, which is as ancient as Democritus, and the system of atoms, had given way to that of the Cartesians, and other mechanists; but is now with great addres<*> retrieved, and improved on, by some of the latest writers, particularly Homberg, the younger Lemery, Gravesande, Boerhaave in his lectures on fire, Black, Crawford, and other chemical philosophers.|

The thing called fire, according to Boerhaave, is a body sui generis, created such ab origine, unalterable in its nature and properties, and not either producible de novo from any other body, nor capable of being reduced into any other body, or of ceasing to be fire. This fire, he contends, is diffused equably every where, and exists alike, or in equal quantity, in all the parts of space, whether void, or possessed by bodies; but that naturally, and in itself, it is perfectly latent and imperceptible; being only discovered by certain effects which it produces, and which are cognizable by our senses. These effects are Heat, light, colour, rarefaction, and burning, which are all indications of fire, as being none of them producible by any other cause: so that whereever we observe any of these, we may safely infer the action and presence of fire. But though the effect cannot be without the cause, yet the fire may remain without any of these effects; any, we mean, gross enough to affect our senses, or become objects of them: and this, he adds, is the ordinary case; there being a concurrence of other circumstances, which are often wanting, necessary to the production of such sensible effects.

The mechanical philosophers, particularly Bacon, Boyle, and Newton, conceive otherwise of Heat; considering it not as an original inherent property of any particular sort of body; but as mechanically producible in any body. The former, in an express treatise De Forma Calidi, from a particular enumeration of the several phenomena and effects of Heat, deduces several general properties of it; and hence he defines Heat, an expansive undulatory motion in the minute particles of the body; by which they tend, with some rapidity, towards the circumference, and at the same time incline a little upwards.

Mr. Boyle, in a Treatise on the Mechanical Origin of Heat and Cold, strongly supports the doctrine of the producibility of Heat, with new observations and experiments; as in the instance of a smith briskly hammering a small piece of iron, which, though cold before, soon becomes exceedingly hot.

This system is also farther supported by Newton, who does not conceive fire as any particular species of body, originally endued with such and such properties. Fire, according to him, is only a body much ignited, that is heated hot, so as to emit light copiously: what else, says he, is red-hot iron but fire? and what else is a burning charcoal but red-hot wood? or flame itself, but red-hot smoke? It is certain that flame is only the volatile part of the fuel heated red-hot, i. e. so hot as to shine; and hence only such bodies as are volatile, that is, such as emit a copious fume, will flame; nor will they flame longer than they have fume to burn. In distilling hot spirits, if the head of the still be taken off, the ascending vapours will catch fire from a candle, and turn into a flame. And in the same manner several bodies, much heated by motion, attrition, fermentation, or the like, will emit lucid fumes, which, if they be copious enough, and the heat sufficiently great, will be flame; and the reason why fused metals do not flame, is the smallness of their fume; this is evident, because spilter, which fumes most copiously, does likewise flame. Add, that all flaming bodies, as oil, tallow, wax, wood, pitch, sulphur, &c, by flaming, waste and vanish into burning smoke. And do not all fixed bodies, when heated beyond a certain degree, emit light, and shine? and is not this emission performed by the vibrating motion of their parts? and do not all bodies, which abound with terrestrial and sulphureous parts, emit light as often as those parts are sufficiently agitated, whether that agitation be made by external fire, or by friction, or percussion, or putrefaction, or by any other cause? Thus, sea water, in a storm; quicksilver agitated in vacuo; the back of a cat, or the neck of a horse, obliquely rubbed in a dark place; wood, fles<*>, and fish, while they putrefy; vapours from putrefying waters, usually called ignes fatui; stacks of moist hay or corn; glow-worms; amber and diamonds by rubbing; fragments of steel struck off with a flint, &c, all emit light. Are not gross bodies and light convertible into one another? and may not bodies receive much of their activity from the particles of light which enter their composition? I know no body less apt to shine than water; and yet water, by frequent distillations, changes into fixed earth, which, by a sufficient Heat, may be brought to shine like other bodies.

Add, that the sun and stars, according to Newton's conjecture, are no other than great earths vehemently heated: for large bodies, he observes, preserve their Heat the longest, their parts heating one another; and why may not great, dense, and fixed bodies, when heated beyond a certain degree, emit light so copiously, as by the emission and reaction of it, and the reflections and refractions of the rays within the pores, to grow still hotter, till they arrive at such a period of Heat as is that of the sun? Their parts also may be farther preserved from fuming away, not only by their fixity, but by the vast weight and density of their atmospheres incumbent on them, thus strongly compressing them, and condensing the vapours and exhalations arising from them. Hence we see warm water, in an exhausted receiver, shall boil as vehemently as the hottest water open to the air; the weight of the incumbent atmosphere, in this latter case, keeping down the vapours, and hindering the ebullition, till it has conceived its utmost degree of Heat. So also a mixture of tin and lead, put on a red-hot iron in vacuo, emits a fume and flame; but the same mixture in the open air, by reason of the incumbent atmosphere, does not emit the least sensible flame.

Thus much for the system of the producibility of Heat.

On the other hand, M. Homberg, in his Essai du Soufre Principe, holds, that the chemical principle or element, sulphur, which is supposed one of the simple, primary, pre-existent ingredients of all natural bodies, is real fire; and consequently that fire is co-eval with body. Mem. de l'Acad. an. 1705.

Dr. Gravesande goes upon much the same principle. According to him, fire enters the composition of all bodies, is contained in all bodies, and may be separated or procured from all bodies, by rubbing them against each other, and thus putting their fire in motion. But fire, he adds, is by no means generated by such motion. Elem. Phys. tom. 2, cap. 1. Heat, in the hot body, he says, is an agitation of the parts of the body, made by means of the fire contained in it; by such agitation a motion is produced in our bodies, which excites the idea of Heat in our minds: so that Heat, in respect of us, is nothing but that idea, and in the hot body nothing but motion. If such motion expel| the fire in right lines, it may give us the idea of light; if in a various and irregular motion, only of Heat.

Lemery, the younger, agrees with these two authors, in asserting this absolute and ingenerable nature of fire; but he extends it farther. Not contented with confining it as an element to bodies, he endeavours to shew, that it is equably diffused through all space; that it is present in all places, even in the void spaces between the bodies, as well as in the insensible interstices between their parts. And this last sentiment falls in with that of Boerhaave above delivered. Mem. de l'Acad. an. 1713.

Philosophers have lately distinguished Heat into Absolute, and Sensible. By Absolute Heat, or fire, they mean that power or element which, when it is in a certain degree, excites in animals the sensation of Heat; and by Sensible Heat, the same power considered in its relation to the effects which it produces: thus, two bodies are said to have equal quantities of sensible Heat, when they produce equal effects upon the mercury in the thermometer; but as bodies of different kinds have different capacities for containing Heat, the absolute Heat in such bodies will be different, though the sensible Heat be the same. Thus, a pound of water and a pound of antimony, of the same temperature, have equal sensible Heat; but the former contains a much greater quantity of absolute Heat than the latter.

M. De Luc has evinced, by a variety of experiments, that the expansions of mercury between the freezing and boiling points of water, correspond precisely to the quantities of absolute Heat applied, and that its contractions are proportionable to the diminution of this element within these limits. And from hence it may be inferred, that if the mercury were to retain its fluid form, its contractions would be proportionable to the decrements of the absolute Heat, though the diminution were continued to the point of total privation. But the comparative quantities of absolute Heat, which are communicated to different bodies, or separated from them, cannot be determined in a direct manner by the thermometer.

Some philosophers have apprehended that the quantities of absolute Heat in bodies, are in proportion to their densities. While others, as Boerhaave, imagined that Heat is equally diffused through all bodies, the densest as well as the rarest, and therefore that the quantities of Heat in bodies are in proportion to their bulk or magnitude: and, at his desire, Fahrenheit attempted to determine the fact by experiment. For this purpose, he took equal quantities of the same fluid, and gave them different degrees of Heat, then upon mixing them intimately together, he found that the temperature of the mixture was a just medium, or arithmetical mean, between the two. But if this experiment be made with water and mercury, in the same circumstances, viz in equal bulks, the result will be different, as the temperature of the mixture will not be a mean between the two, but always nearer to that of the water than to the quicksilver; so that, when the water is the hotter, the temperature of the mixture is above the mean, and below it when the water is the colder. And from experiments of this kind it has been inferred, that the comparative quantities of the absolute Heat<*> of these fluids, are reciprocally proportional to the changes which are produced in their sensible Heats, when they are mixed together at different temperatures: and this fact has been publicly taught, for several years, by Dr. Black, and Dr. Irvine, in the universities of Edinburgh and Glasgow. This rule however does not apply to those substances which, in mixture, excite sensible Heat by chemical action.

From the experiments and reasoning employed by Dr. Crawford, it more fully appears, that the quantities of absolute Heat in different bodies, are not as their densities; or that equal weights of heterogeneous substances, as air and water, having the same temperature<*> may contain unequal quantities of absolute Heat: he also shews, that if phlogiston be added to a body, a quantity of the absolute Heat of that body will be extricated; and if the phlogiston be separated again, an equal quantity of Heat will be absorbed. So that Heat and phlogiston appear to be two opposite principles in nature. But this ingenious writer has not presumed absolutely to decide the question that has been long agitated, whether Heat be a substance or a quality.— He inclines to the former opinion however, and observes, that if we adopt the opinion, that Heat is a distinct substance, or an element sui generis, the phenomena will be found to admit of a simple and obvious interpretation, and to be perfectly agreeable to the analogy of nature. See Crawford's Experiments and Observations<*> on Animal Heat and the Inflammation of Combustible Bodies.

Animal Heat. The Heat of animals is very various, both according to the variety of their kinds, and the difference of the seasons: accordingly, zoologists have divided them into hot and cold blooded, reckoning those to be hot that are near or above our own temperature, and all others cold whose Heat is below ours, and consequently affect us with the sense of cold; thus making the human species a medium between the hot and cold blooded animals, or at least the lowest order of the hot blooded.

The Heat of the human body, in its natural state, according to Dr. Boerhaave, is such as to raise the mercury in the thermometer to 92° or at most to 94°; and Dr. Pitcairn makes the heat of the human skin the same. Indeed it is evident that different parts of the human body, and its different states, as well as the different seasons, will make it shew of different temperatures. Thus, by various experiments at different times, the Heat of the human body is made various by the following authors:

Boerhaave and Pitcairn92°
Amontons91, 92, or 93
Sir Isaac Newton95 1/2
Fahrenheit and Musschenbro<*>k, the blood,96
Dr. Martine, the skin97 or 98
-----------, the urine99
Dr. Hales, the skin97
---------, the urine103
Mr. John Hunter, under his tongue,97
---------------, in his rectum98 1/2
---------------, his urethra at 1 inch,92
                            at 2 inches,93
                            at 4 inches,9<*>
        the ball of the thermom. at the bulb}97
             of the urethra

For the powers of animals to bear various degrees of Heat, see the Philof. Trans. vol. 65, 68, &c.

There is hardly any subject of philosophical investigation that has afforded a greater variety of hypotheses, conjectures, and-experiments, than the cause of animal Heat. The first opinion which has very generally obtained, is, that the Heat of animal bodies is owing to the attrition between the arteries and the blood. All the observations and reasoning brought in favour of this opinion however, only shew that the Heat and the motion of the arteries are generally proportional to each other; without shewing which is the cause, and which the effect; or indeed that either is the cause or effect of the other, since both may be the effects of some other cause.

Dr. Douglas, in his Essay on the Generation of Heat in animals, ascribes it solely to the friction of the globules of blood in their circulation through the capillary vessels.

Another opinion is, that the lungs are the fountain of Heat in the human body: and this opinion is supported by much the same sort of arguments as the former, and seemingly to little better purpose.

A third opinion is, that the cause of animal Heat is owing to the action of the solid parts upon one another. And as the heart and arteries move most, it has been thought natural to expect that the Heat should be owing to this motion. But even this does not seem very plausible, from the following considerations: 1st, The moving parts, however we term them solid, are neither hard nor dry; which two conditions are absolutely requisite to make them sit to generate Heat by attrition. 2d, None of their motions are swift enough to promise Heat in this way. 3d, They have but little change of surface in their attritions. And 4thly, The moveable fibres have fat, mucilage, or liquors everyway surrounding them, to prevent their being destroyed, or heated by attrition.

A fourth cause assigned for the Heat of our bodies, is that process by which our aliment and fluids are perpetually undergoing some alteration. And this opinion is chiefly supported by Dr. Stevenson, in the Edinburgh Medical Essays, vol. 5, art. 77.

The late ingenious Dr. Franklin inclines to this opinion, when he says, that the fluid fire, as well as the fluid air, is attracted by plants in their growth, and becomes consolidated with the other materials of which they are formed, and makes a great part of their substance; that when they come to be digested, and to undergo a kind of fermentation in the vessels, part of the fire, as well as part of the air, recovers its fluid active state again, and diffuses itself on the body digesting and separating it; &c. Exper. and Obs. on Electricity, p. 346.

Dr. Mortimer thinks the Heat of animals explicable from the phosphorus and air they contain. Phosphorus exists, at least in a dormant state, in animal fluids; and it is also known that they all contain air; it is therefore only necessary to bring the phosphoreal and aereal particles into contact, and Heat must of consequence be generated; and were it not for the quantity of aqueous humours in animals, fatal accensions would frequently happen. Philos. Trans. number 476.

Dr. Black supposes, that animal Heat is generated altogether in the lungs, by the action of the air on the principle of inflammability, and is thence diffused over the rest of the body by means of the circulation. But Dr. Leslie urges several arguments against this hypothesis, tending to shew that it is repugnant to the known laws of the animal machine; and he advances another hypothesis instead of it, viz, that the subtle principle by chemists termed phlogiston, which enters into the composition of natural bodies, is in consequence of the actio<*> of the vascular system gradually evol<*>ed through every part of the animal machine, and that during this evolution Heat is generated. This opinion, he candidly acknowledges, was first delivered by Dr. Duncan of Edinburgh; and that something similar to it is to be found in Dr. Franklin's works, and in a paper of Dr. Mortimer's in the Philos. Trans.

The last hypothesis we shall mention, is the very plausible one of Dr. Crawford, lately published in his Experiments and Observations on Animal Heat. This isgenious gentleman has inferred, from a variety of experiments, that Heat and phlogiston, so far from being connected, as most philosophers have imagined, act in some measure in opposition to each other. By the action of Heat on bodies, the force of their attraction of phlogiston is diminished, and by the action of phlogiston, a part of their absolute Heat is expelled. He has also demonstrated, that atmospherical air contains a greater quantity of absolute Heat than the air which is expired from the lungs of animals: he makes the proportion of the absolute Heat of atmospherical air to that of fixed air, as 67 to 1; and the Heat of dephlogisticated air to that of atmospherical air as 4.6 to 1; and observing that Dr. Priestley has proved, that the power of this dephlogisticated air in supporting animal life, is 5 times as great as that of atmospherical air, he concludes that the quantity of absolute Heat contained in any kind of air fit for respiration, is very nearly in proportion to its purity, or to its power of supporting animal life; and since the air exhaled by respiration, is found to contain only the 67th part of the Heat which was contained in the atmospherical air, previous to inspiration, it is very reasonably inferred, that the latter must necessarily deposit a very great proportion of its absolute Heat in the lungs. Dr. Crawford has also shewn, that the blood which passes from the lungs to the heart, by the pulmonary vein, contains more absolute Heat than that which passes from the heart to the lungs, by the pulmonary artery; the absolute Heat of florid arterial blood being to that of venous blood, as 11 1/2 to 10: therefore, since the blood which is returned by the pulmonary vein to the heart has the quantity of its absolute Heat increased, it must have acquired this Heat in its passage through the lungs; so that in the process of respiration a quantity of absolute Heat is separated from the air, and absorbed by the blood. Dr. Priestley has also proved, that in respiration, phlogiston is separated from the blood, and combined with air.

This theory however has been contested and disputed, and, it has been said, Dr. Crawford's experiments repeated, with contrary results. Though no regular and systematical theory has yet been formed in its stead.

Heat of Combustible and Inflammable Bodies. Dr. Crawford's theory with respect to the inflammation of| combustible bodies, is founded on the same principles as his doctrine concerning the Heat of animals. According to him, the Heat which is produced by combustion, is derived from the air, and not from the inflammable body. Inflammable bodies, he says, abound with phlogiston, and contain little absolute Heat: the atmosphere, on the contrary, abounds with absolute Heat, and contains little phlogiston. In the process of inflammation, the phlogiston is separated from the inflammable body, and combined with the air; the air is phlogisticated, and gives off a great proportion of its absolute Heat, which, when extricated suddenly, bursts forth into flame, and produces an intense degree of sensible Heat. And since it appears by calculation, that the Heat produced by converting atmospherical into fixed air, is such, if it were not dissipated, as would be sufficient to raise the air so changed, to more than 12 times the Heat of red-hot iron, it follows, that in the process of inflammation a very great quantity of Heat is derived from the air. But, on the contrary, no part of the Heat can be derived from the combustible body; because this body, during the inflammation, being deprived of its phlogiston, undergoes a change similar to that of the blood by the process of respiration, in consequence of which its capacity of containing Heat is increased; and therefore it will not give off any part of its absolute Heat, but, like the blood in its passage through the lungs, it will absorb Heat.

A similar theory of Heat has lately been published by Mr. Elliot. See his Philosophical Observations on the senses of Vision and Hearing; to which is added an Essay on Combustion and Animal Heat. 8vo, 1780.

Heat, in Geography, is that which relates to the earth. There is a great variety in the Heat of different places and seasons. Naturalists have commonly laid it down, that the nearer any place is to the centre of the earth, the hotter it is found; but this does not hold strictly true. And if it were, the effect might be otherwise accounted for, and more satisfactorily, than from their imagined central fire.

Mr. Boyle, who had been at the bottom of some mines himself, with more probability fuspects that this degree of Heat, at least in some of them, may arise from the peculiar nature of the minerals there produced. And he instances a mineral of the vitriolic kind, dug up in large quantities, in several parts of England, which, by the bare effusion of common water, will grow so hot as almost to take fire. To which may be added, that such places, in the bowels of the earth, usually feel hot, from the confined and stagnant state of the air in them, in which the heat is retained, through the want of a current or change of air to carry the Heat off.

On the other hand, on ascending high mountains, the air grows more and more cold and piercing. Thus, the tops of the Pike of Teneriffe, the Alps, and several other mountains, even in the most sultry countries, are found always invested with snow and ice, which the Heat is never sufficient to thaw. In some of the mountains of Peru there is no such thing as running water, but all ice: plants vegetate a little about the bottom of the mountains, but near the top no vegetable can live, for the intenseness of the cold. This effect is attributed to the thinness of the air, and the little surface of the earth there is to reflect the rays, as well as the great distance of the general surface of the earth which reflects the rays back into the atmosphere.

As to the diversity in the Heat of different climes and seasons, it arises <*>rom the different angles under which the sun's rays strike upon the surface of the earth. In the Philos. Trans. Abr. vol. 2, p. 165, Dr. Halley has given a computation of this Heat, on the principle, that the simple action of the sun's rays, like other impulses or strokes, is more or less forcible, according to the <*>ines of the angles of incidence, or to the sines of the sun's altitudes, at different times or places.

Hence it follows, that, the time of continuance, or the sun's shining on any place, being taken for a basis, and the sines of the sun's altitudes perpendicularly erected upon it, and a curve line drawn through the extremities of those perpendiculars, the area thus comprehended will be proportional to the collection of all the Heat of the sun's beams in that space of time.

Hence it will likewise follow, that at the pole, the collection of all the Heat of a tropical day, is proportional to the rectangle or product of the fine of 23 1/2 degrees into 24 hours, or the circumference of a circle, or as <*>/10 into 12 hours, the sine of 23 1/2 degrees being nearly 4/10 of radius. Or the polar Heat will be equal to that of the sun continuing 12 hours above the horizon at 53 degrees height; and the sun is not 5 hours more elevated than this under the equinoctial.

But as it is the nature of Heat to remain in the subject, after the luminary is removed, and particularly in the air, under the equinoctial the 12 hours absence of the sun abates but little from the effect of his Heat in the day; but under the pole, the long absence of the sun for 6 months has so chilled the air, that it is in a manner frozen, and after the sun has risen upon the pole again, it is long before his beams can make any impression, being obstructed by thick clouds and fogs.

From the foregoing principle Dr. Halley computes the following table, exhibiting the Heat to every 10th degree of latitude, for the equinoctial and tropical sun, and from which an estimate may easily be made for the intermediate degrees.

Lat.Sign that the Sun is in.
<*> or <*><*><*>

From the same principles, and table, also are deduced the following corollaries, viz,|

1, That the equatorial Heat, when the sun becomes vertical, is as twice the square of the radius.—2, That at the equator the Heat is as the sine of the sun's declination.—3, That in the frigid zones, when the sun sets not, the Heat is as the circumference of a circle into the sine of the altitude at 6: And consequently that in the same latitude, these aggregates of Heat are as the fines of the sun's declination; and at the same declination of the sun, they are as the sines of the latitudes; and generally they are as the sines of the latitudes into the sines of declination.—4, That the equatorial day's Heat is everywhere as the cosine of the latitude.—5, In all places where the sun sets, the difference between the summer and winter Heats, when the declinations are contrary, is equal to a circle into the sine of the altitude at 6, in the summer parallel; and consequently those differences are as the rectangles of the sines of the latitude and declination. —6, The tropical sun has the least force of any at the equator; and at the pole it is greatest of all.

Many objections have been urged against this theory of Dr. Halley. Some have objected, that the effect of the sun's Heat is not in the simple, but in the duplicate ratio of the sines of the angles of incidence; like the law of the impulse of fluids. And indeed, the quantity of the sun's direct rays received at any place, being evidently as the sine of the angle of incidence, or of the sun's altitude, if the Heat be also proportional to the force with which a ray strikes, like the mechanical action or impulse of any body, then it will follow that the Heat must be in the compound ratio of both, that is, as the square of the sine of the sun's altitude. But this last principle is here only assumed gratis, as we do not know a priori that the Heat is proportional to the force of a striking body; and it is only experiment that can determine this point.

It is certain that Heat communicated by the sun to bodies on the earth, depends also much upon other circumstances beside the direct force of his rays. These must be modified by our atmosphere, and variously reflected and combined by the action of the earth's surface itself, to produce any remarkable effects of Heat. So that if it were not for these additional circumstances, it is probable the naked Heat of the sun would not be very sensible.

Dr. Halley himself was well apprised, that many other circumstances, besides the direct force of the sun's rays, contributed to augment or diminish the effect of this, and the Heat resulting from it, in different climates; and therefore no calculation, formed on the preceding theory, can be supposed to correspond exactly with observation and experiment. It has also been objected that, according to the foregoing theory, the greatest Heat in the same place should be at the summer solstice, and the most extreme cold at the winter solstice; which is contrary to experience. To this objection it may be replied, that Heat is not produced in bodies by the sun instantaneously, nor do the effects of his Heat cease immediately when his rays are withdrawn; and therefore those parts which are once heated, retain the Heat for some time; which, with the additional Heat daily imparted, makes it continue to increase, though the sun declines from us: and this is the reason why July is hotter than June, although the sun has withdrawn from the summer tropic: as we also sind it is generally hotter at one, two, or three in the afternoon, when the sun has declined towards the west, than at noon, when he is on the meridian. As long as the heating particles, which are constantly received, are more numerous than those which fly away or lose their force, the Heat of bodies must continually increase. So, after the sun has left the tropic, the number of particles, which Heat our atmosphere and earth, constantly increases, because we receive more in the day than we lose at night, and therefore our Heat must also increase. But as the days decrease again, and the action of the sun becomes weaker, more particles will fly off in the night time than are received in the day, by which means the earth and air will gradually cool. Farther, those places which are well cooled, require time to be heated again; and therefore January is mostly colder than December, although the sun has withdrawn from the winter tropic, and begun to emit his rays more perpendicularly upon us.

But the chief cause of the difference between the Heat of summer and winter is, that in summer the rays fall more perpendicularly, and pass through a less dense part of the atmosphere; and therefore with greater force, or at least in greater number in the same place: and besides, by their long continuance, a much greater degree of Heat is imparted by day than can fly off by night.

For the calculations and opinions of several other philosophers on this head, see Keill's Astron. Lect. 8; Ferguson's Astron. chap. 10; Long's Astron. § 777; Memo. Acad. Scienc. 1719.

As to the temperature or Heat of our atmosphere, it may be observed that the mercury seldom falls under 16° in Fahrenheit's thermometer; but we are apt to reckon it very cold at 24°, and it continues coldish to 40° and a little above. However, such colds have been often known as bring it down to 0°, the beginning of the scale, or nearly the cold produced by a mixture of snow and salt, often near it, and in some places below it. Thus, the degree of the thermometer has been observed at various times and places as follows:

Pensylvania40°  0′1732
Paris48 501709 & 17108
Leyden52 1017295
Utrecht52  8----4
London51 311709 & 17100
Copenhagen55 4317090
Upsal59 561732-1
Petersburg59 56-----28
Torneo65 511736-7-33
Hudson's Bay52 241775-37

The middle temperature of our atmosphere is about 48°, being nearly a medium of all the seasons. The French make it somewhat higher, reckoning it equal to the cave of their royal observatory, or 53°. In cold countries, the air is found agreeable enough to the inhabitants while it is between 40 and 50°. In our climate we are best pleased with the heat of the air from 50 to 60° while in the hot countries the air is gene rally at a medium about 70°. With us, the air is not| reckoned warm till it arrives at about 64°, and it is very warm and sultry at 80°. It is to be noted that the foregoing observations are to be understood of the state of the air in the shade; for as to the Heat of bodies acted upon by the direct rays of the sun, it is much greater: thus, Dr. Martine found dry earth heated to above 120°; but Dr. Hales found a very hot sun-shine Heat in 1727 to be about 140°; and Musschenbroek once observed it so high as 150°; but at Montpelier the sun was so very hot, on one day in the year 1705, as to raise M. Amontons's thermometer to the mark of boiling water itself, which is our 212°.

It appears from the register of the thermometer kept at London by Dr. Heberden for 9 years, viz, from the end of 1763 to the end of 1772, that the mean Heat at 8 in the morning was 47°.4; and by another register kept at Hawkhill, near Edinburgh, that the mean Heat in that place, during the same period of time, was 46°. Also by registers kept in London and at Hawkhill, for the three years 1772, 1773, 1774, it appears, that the mean Heat of these three years in London, at 8 in the morning, was 48°.5, and at 2 in the afternoon 56°, but the mean of both morning and afternoon 52°.2; while the mean Heat at Hawkhill for the same time,

at 8 in the morning was45°.4
and at 2 in the afternoon50.1
and the mean of both47.7.
The mean Heat of springs near Edinburgh seems to be 47°, and at London 51°. Philos. Trans. vol. 65, art. 44.

Lastly, from the meteorological journals of the Royal Society, published in the Philos. Trans. it appears that the mean heights of the thermometer, for the whole years, kept without and within the house, are as below:

Therm. WithoutTherm. Within
For 1775  51.552.7
1776  51.152.9
1777  51.053.0
1778  52.053.1
mean of all51.452.9

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Entry taken from A Mathematical and Philosophical Dictionary, by Charles Hutton, 1796.

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HARRIOT (Thomas)
HAYES (Charles, Esq.)