POINT

, a term used in various arts and sciences.

, in Architecture. Arches of the third Point, and Arches of the fourth Point. See Arches.

, in Astronomy, is a term applied to certain parts or places marked in the heavens, and distinguished by proper epithets.

The four grand points or divisions of the horizon, viz, the east, west, north, and south, are called the Cardinal Points.—The zenith and nadir are the Vertical Points. —The Points where the orbits of the planets cut the plane of the ecliptic, are called the Nodes.—The Points where the ecliptic and equator intersect, are called the Equinoctial Points. In particular, that where the sun ascends towards the north pole is called the Vernal Point; and that where he descends towards the south, the Autumnal Point — The highest and lowest Points of the ecliptic are called the Solstitial Points. Particu larly, the former of them the Estival or Summer Point; the latter, the Brumal or Winter Point.

, in Electricity, are those acute terminations of bodies which facilitate the passage of the electrical fluid either from or to such bodies.

Mr. Jallabert was probably the first person who observed that a body pointed at one end, and round at the other, produced different appearances upon the same body, according as the pointed or round end was presented to it. But Dr. Franklin first observed and evinced the whole effect of pointed bodies, both in drawing and throwing off electricity at greater distances than other bodies could do it; though he candidly acknowledges, that the power of Points to throw off the electric sire was communicated to him by his sriend Mr. Thomas Hopkinson.

Dr. Franklin electrified an iron shot, 3 or 4 inches in diameter, and observed that it would not attract a thread when the Point of a needle, communicating with the earth, was presented to it; and he found it even impossible to electrify an iron shot when a sharp needle lay upon it. This remarkable property, possessed by pointed bodies, of gradually and silently receiving or throwing off the electric fluid, has been evinced by a variety of other familiar experiments.

Thus, if one hand be applied to the outside coating of a large jar fully charged, and the Point of a needle held in the other, be directed towards the knob of the jar, and moved gradually near it, till the Point of the needle touch the knob or ball, the jar will be entirely discharged, so as to give no shock at all, or one that is hardly sensible. In this case the Point of the needle has gradually and silently drawn away the superabundant electricity from the electrified jar.

Farther, if the knob of a brass rod be held at such a distance from the prime conductor, that sparks may easily escape from the latter to the former, whilst the machine is in motion; then if the Point of a needle be presented, though at twice the distance of the rod from the conductor, no more sparks will be seen passing to the rod. When the needle is removed, the sparks will be seen; but upon presenting it again, they will again disappear. So that the Point of the needle draws off silently almost all the fluid, which is thrown by the cylinder or globe of the machine upon the prime conductor. This experiment may be varied, by fixing the needle upon the prime conductor with the point upward; and then, though the knob of a discharging rod, or the knuckle of the singer, be brought very near the prime conductor, and the excitation be very strong, little or no spark will be perceived.

The influence of points is also evinced in the amusing experiment, commonly called the electrical horse-race, and many others. See Thunder-house.

The late Mr. Henly exhibited the efficacy of pointed bodies, by suspending a large bladder, well blown, and covered with gold, silver, or brass leaf, by means of gum-water, at the end of a silken thread 6 or 7 feet long, hanging from the cieling of a room, and electrifying the bladder by giving it a strong spark with the knob of a charged bottle: upon presenting to it the knob of a wire, it caused the bladder to move towards the knob, and when nearly in contact gave it a spark, thus discharging its electricity. By giving the bladder another charge, and presenting the Point of a needle to it, the bladder was not attracted by the Point, but| rather receded from it, especially when the needle was suddenly presented towards it.

But experiments evincing the efficacy of pointed bodies for silently receiving or throwing off the electric fluid, may be infinitely diversified, according to the fancy or convenience of the electrician.

It may be observed, that in the case of points throwing off or receiving electricity, a current of air is sensible at an electrified Point, which is always in the direction of the Point, whether the electricity be positive or negative. A fact which has been well ascertained by many electricians, and particularly by Dr. Priestley and Sig. Beccaria. The former contrived to exhibit the influence of this current on the flame of a candle, presented to a pointed wire, electrified negatively, as well as positively. The blaft was in both cases alike, and so strong as to lay bare the greatest part of the wick, the flame being driven from the Point; and the effect was the same whether the electric fluid issued out of the Point or entered into it. He farther evinced this phenomenon by means of thin light vanes; and he found, as Mr. Wilson had before observed, that the vanes would not turn in vacuo, nor in a close unexhausted receiver where the air had no sree circulation. And in much the same manner, Beccaria exhibited to sense the influence of the wind or current of air driven from points.

As to the Theory of the phenomena of Points, these are accounted for in a variety of ways, by different authors, though perhaps by none with perfect satisfaction. See Franklin's writings on Electricity; Lord Mahon's Principles of Electricity, 1779; Beccaria's Artisicial Electricity, 1776, pa. 331; and Priestlcy's History of Electricity, vol. 2, pa. 191, edit. 1775.

As to the Application of the doctrine of Points; it may be observed that there is not a more important fact in the history of electricity, than the use to which the discovery of the efficacy of pointed bodies has been applied.

Dr. Franklin, having ascertained the identity of electricity and lightning, was presently led to propose a cheap and easy method of securing buildings from the damage of lightning, by fixing a pointed metal rod higher than any part of the building, and communicating with the ground, or with the nearest water. And this contrivance was actually executed in a variety of cases; and has usually been thought an excellent preservative against the terrible effects of lightning.

Some few instances however having occurred, in which buildings have been struck and damaged, though provided with these conductors; a controversy arose with regard to their expediency and utility. In this controversy Mr. Benjamin Wilson took the lead, and Dr. Musgrave, and some few other electricians, the least acquainted with the subject, concurred with him in their opposition to pointed elevated conductors. These alledge, that every Point, as such, solicits the lightning, and thus contributes not only to increase the quantity of every actual discharge, but also frequently to occasion a discharge when it might not otherwise have happened: whereas, say they, if instead of pointed conductors, those with blunted terminations were used, they would as effectually answer the purpose of conveying away the lightning safely, without the same tendency to increase or invite it. Accordingly, Mr. Wilson, in a letter to the marquis of Rockingham (Philos. Trans. vol. 54, art. 44), expresses his opinion, that, in order to prevent lightning from doing mischief to high buildings, large magazines, and the like, instead of the elevated external conductors, that, on the inside of the highest part of such building, and within a foot or two of the top, it may be proper to fix a rounded bar of metal, and to continue it down along the side of the wall to any kind of moisture in the ground.

On the other hand, it is urged by the advocates for pointed conductors, that Points, instead of increasing an actual discharge, really prevent a discharge where it would otherwise happen, and that blunted conductors tend to invite the clouds charged with lightning. And it seems to be a certain fact, that though a sharp Point will draw off a charge of electricity silently at a much greater distance than a knob, yet a knob will be struck with a full explosion or shock, the charge being the same in both cases, at a greater distance than a sharp Point.

The efficacy of pointed bodies for preventing a stroke of lightning, is ingeniously explained by Dr. Franklin in the following manner:—An eye, he says, so situated as to view horizontally the underside of a thunder-cloud, will see it very ragged, with a number of separate fragments or small clouds one under another; the lowest sometimes not far from the earth. These, as so many stepping-stones, assist in conducting a stroke between a cloud and a building. To represent these by a<*> experiment, he directs to take two or three locks of fine loose cotton, and connect one of them with the prime conductor by a fine thread of 2 inches, another to that, and a third to the second, by like threads, which may be spun out of the same cotton. He then directs to turn the globe, and says we shall see these locks extending themselves towards the table, as the lower small clouds do towards the earth; but that, on presenting a sharp Point, erect under the lowest, it will shrink up to the second, the second up to the sirst, and all together to the prime conductor, where they will continue as long as the Point continues under them. May not, he adds, in like manner, the small electrified clouds, whose equilibrium with the earth is soon restored by the Point, rise up to the main body, and by that means occasion so large a vacancy, as that the grand cloud cannot strike in that place? Letters, pa. 121.

Mr. Henly too, as well as several other persons, with a view of determining the question, whether Points or knobs are to be preferred for the terminations of conductors, made several experiments, shewing in a variety of instances, the efsicacy of Points in silently drawing off the electricity, and preventing strokes which would happen to knobs in the same situation. Philos. Trans. vol. 64, part 2, art. 18. See also Thunder- Hous.

Indeed it has been universally allowed, that in cases where the quantity of electricity, with which thunderclouds are charged, is small, or when they move slowly in their passage to and over a building, pointed conductors, which draw off the electrical fluid silently, within the distance at which rounded ends will explode, will gradually exhaust them, and thus contribute to prevent a stroke and preserve the buildings to which they are annexed.|

But it has been said by those who are averse to the use of such conductors, that if clouds, of great extent, and highly electrified, should be driven directly over them with great velocity, or if a cloud hanging directly over buildings to which they are annexed, suddenly receives a charge by explosion from another cloud at a distance, so as to enable it instantly to strike into the earth, these pointed conductors must take the explosion; on account of their greater readiness to admit electricity at a much greater distance than those that are blunted, and in proportion to the difference of that striking distance, do mischief instead of good: and therefore, they add, that such pointed conductors, though they may be sometimes advantageous, are yet at other times prejudicial: and that, as the purpose for which conductors are fixed upon buildings, is not to protect them from one particular sort of clouds only, but if possible from all, it cannot be advisable to use that kind of conductors which, if they diminish danger on the one hand, will increase it on the other. Besides, it is alleged, that if pointed conductors are at<*> tended with any the slightest degree of danger, that danger must be considerably augmented by carrying them high up into the air, and by fixing them upon every angle of a building, and by making them project in every direction. Such is the reasoning of Dr. Musgrave: see his paper in the Philos. Trans. vol. 68, part 2, art. 36.

Mr. Wilson too, dissenting from the report of a committee of the Royal Society, appointed to inspect the damage done by lightning to the house of the Board of Ordnance, at Purfleet, in 1777, was led to justify his dissent, and to disparage the use of pointed and elevated conductors, by means of a magnificent apparatus he constructed, with which he might produce effects si<*>ilar to those that had happened in the case referred to the consideration and decision of the committee. With this view he procured a model of the Board-house at Purfleet, resembling it as nearly as possible in every essential appendage, and furnished with conductors of different lengths and terminations. And to construct a substitute for a cloud, he joined together the broad rims of 120 drums, forming together a cylinder of 155 feet in length, and above 16 inches in diameter; and this immense cylinder, of about 600 square feet of coated surface, was connected occasionally with one end of a wire 4800 feet long. As this bulky apparatus, representing the thunder-cloud, could not conveniently be put in motion, he contrived to accomplish the same end by moving the model of the building, with a velocity answering to that of the cloud, which he states, at a moderate computation, to be about 4 or 5 miles an hour. This apparatus was charged by a machine with one glass cylinder, about 10 or 11 feet from its nearest end; and the whole of the apparatus was disposed in the great room of the Pantheon, and applied to use in a variety of experiments. But it is impossible within the limits of this article to do justice to Mr. Wilson's experiments, or to the inferences which he deduces from them. Suffice it just to observe, that most of his experiments, in which the model of the house, which was passed swiftly under the artificial cloud, and having annexed to it either the pointed or blunt conductors at the same or different heights, were intended to shew, that pointed conductors are struck at a greater distance, and with a higher elevation, than the blunted ones: and from all his experiments made with pointed and rounded conductors, provided the circumstances be the same in both, he infers, that the rounded ones are much the safer of the two; whether the lightning proceeds from one cloud or from several; that those are still safer which rise little or nothing above the highest part of the building; and that this safety arises from the greatest resistance exerted at the larger surface. See Philos. Trans. for 1778, pa. 232.

The committee of the Royal Society however, which was composed of nine of the most distinguished electricians in the kingdom, and to whom was referred the consideration of the most effectual method of securing the powder-magazines at Purfleet against the effects of lightning, express their united opinion, that elevated sharp rods, constructed and disposed in the manner which they direct, are preferable to low conductors terminated in rounded ends, knobs, or balls of metal; and that the experiments and reasonings, made and alleged to the contrary by Mr. Wilson, are inconclusive.

Mr. Nairne also, in order to obviate the objections of Mr. Wilson and others, and to vindicate the preference generally given to high and pointed conductors, constructed a much more simple apparatus than that of Mr. Wilson, with which he made a number of well-designed and well-conducted experiments, which seem to prove the point as far as it is capable of being proved by an artificial electrical apparatus. From these last experiments it appears, that though the point was struck by means of a swift motion of the artificial cloud, yet a small ball of 3 tenths of an inch diameter was struck farther off than the Point, and a larger ball at a much greater distance than either, even with the swiftest motion. Upon the whole, Mr. Nairne seems to be justi<*>ied in preferring elevated pointed conductors; next to them, those that are pointed, though they rise but little above the highest part of a building; and after them, those that are terminated in a ball, and placed even with the highest part of the building. See Philos. Trans. 1778, pa. 823.

On the other part, Dr. Musgrave, not yet satisfied, gave in another paper, being “Reasons for dissenting from the Report of the Committee appointed to consider of Mr. Wilson's Experiments; including Remarks on some Experiments exhibited by Mr. Nairne;” which is inserted, by mistake, before Mr. Nairne's paper, being at pa. 801 of the same volume.

And farther, Mr. Wilson has another paper, on the same subject, at pa. 999 of the same vol. of Philos. Trans. for 1778, entitled, “New Experiments upon the Leyden Phial, respecting the termination of conductors;” repeating and asserting his former objections and reasonings.

In the Philos. Trans. too for 1779, pa. 454, Mr. William Swift has a paper, farther prosecuting this subject; making various experiments with simple and ingenious machinery, with models of houses and clouds, and with various sorts of conductors. From the experiments he infers in general, that “the whole current| of these experiments tends to shew the preference of Points to balls, in order to diminish and draw off the electric matter when excited, or to prevent it from accumulating; and consequently the propriety or even necessity of terminating all conductors with Points, to make them useful to prevent damage to buildings from lightning. Nay the very construction of all electrical machines, in which it is necessary to round all the parts, and to avoid making edges and points which would hinder the matter from being excited, will, J imagine, on reflection, be another corroborating proof of the result of the experiments themselves.”

There were other communications made to the Royal Society upon the important subject of conductors, some of which were received, and others rejected. Upon the whole, this contest turned out one of the most extraordinary that ever was agitated in the Society; producing the most remarkable disputes, differences, and strange consequences, that ever the Society experienced since it had existence; consequences which manifested themselves in various instances for many years after, and which continue to this very day. All which, with the various secret springs and astonishing intrigues, may probably be given to the public on some other occasion.

, in Geometry, according to Euclid, is that which has no parts, or is indivisible; being void of all extension, both as to length, breadth, and depth.

This is what is otherwise called the Mathematical Point, being the intersection of two lines, and is only conceived by the imagination; yet it is in this that all magnitude begins and ends; the extremes of a line being Points; the extremes of a surface, Lines; and the extremes of a solid, Surfaces. And hence some define a Point, the inceptive of magnitude.

Proportion of Mathematical Points. It is a popular maxim, that all infinites are equal; yet is the maxim false, whether of quantities infinitely great, or infinitely little. Dr. Halley instances in several infinite quantities which are in a finite proportion to each other; and some that are infinitely greater than others. See INFINITE Quantity.

And the same is shewn by Mr. Robarts, of infinitely small quantities, or mathematical Points. He demonstrates, for instance, that the Points of contact between circles and their tangents, are in the subduplicate ratio of the diameters of the circles; that the Point of contact between a sphere and a plane is insinitely greater than between a circle and a line; and that the Points of contact in spheres of different magnitudes, are to each other as the diameters of the spheres. Philos. Trans. vol. 27, pa. 470.

Conjugate Point, is used for that Point into which the conjugate oval, belonging to some kind of curves, vanishes. Maclaurin's Alg. pa. 308.

Point of Contrary Flexure, &c. See Inflexion, Retrogradation or Retrogression, &c, of curves.

Points of the Compass, or Horizon, &c, in Geography and Navigation, are the Points of division when the whole circle, quite around, is divided into 32 equal parts. These Points are therefore at the distance of the 32d part of the circle, or 11° 15′, from each other; hence 5° 37′ 1/2 is the distance of the half points, and 2° 48′ 3/4 is the distance of the quarter Points. See Compass. The principal of these are the four cardinal Points, east, west, north and south.

Point is also used for a cape or headland, jutting ont into the sea.——The seamen say two Points of land are one in another, when they are in a right line, the one behind the other.

, in Optics. As the

Point of Concourse or Concurrence, is that in which converging rays meet; and is usually called focus.

Point of Dispersion, Incidence, Reflection, Refraction, and Radiant Point. See these several articles.

, in Perspective, is a term used for various parts or places, with regard to the perspective plane. As, the

Point of Sight, or of the eye, called also the Principal Point, is the Point on a plane where a perpendicular from the eye meets it. See Perspective.

Some authors, however, by the Point of Sight, or Vision, mean the Point where the eye is actually placed, and where all the rays terminate. See Perspective.

Point of Distance, is a Point in a horizontal line, at the same distance from the principal Point as the eye is from the same. See Perspective.

Third Point, is a Point taken at discretion in the line of distance, where all the diagonals meet that are drawn from the divisions of the geometrical plane.

Objective Point, is a Point on a geometrical plane, whose representation on the perspective plane is required.

Accidental Point, and Visual Point. See ACCIDENTAL and Visual.

Point of View, with regard to Building, Painting, &c, is a Point at a certain distance from a building, or other object, where the eye has the most advantageous view or prospect of the same. And this Point is usually at a distance equal to the height of the building.

, in Physics, is the smallest or least sensible object of sight, marked with a pen, or point of a compass, or the like. This is popularly called a Physical Point, and of such does all physical magnitude consist.

Point-Blanc, Point-Blank, in Gunnery, denotes the horizontal or level position of a gun, or having its muzzle neither elevated nor depressed. And the Pointblanc range, is the distance the shot goes, before it strikes the level ground, when discharged in the horizontal or Point-blanc direction. Or sometimes this means the distance the ball goes horizontally in a straightlined direction.