The opinions of the ancients concerning the caufe of the diverfity of colours, were various and uncertain. This matter was firft rationally accounted for by Sir Ifaac Newton. He discovered that the rays of light, which to us appear to be perfectly homogeneous, and white, are compofed of no fewer than feven different colours; red, orange, yellow, green, blue, purple, and violet or indigo; and that bodies appear of different colours, as they have the property of reflecting fome rays more powerfully than others. Hence a body which is of a black colour, instead of reflecting, abforbs all, or the greatest part of the rays which fall upon it; whence a black cloth imbibes more heat than any other; and, on the contrary, a body which appears white, reflects the greatest part of the rays indifcriminately, without feparating the one from the other. Thus if ftreaks of all the feven colours be intermingled on the surface of a table, and the table made to turn round quickly, it will appear white. Without light vegetables would have no colour, but would all appear white.

The feven different colours of the fun-beams are called Primary colours; and by compounding any two of them, as red and yellow, or yellow and blue, the intermediate colours, as orange or green, may be produced.

All these colours are confpicuous in the rainbow, and are beautifully exemplified by making the fun-beams pafs through a glafs phial, exactly cylindrical, full of water, or through a glafs prism into a dark room.

The ancients believed that water was drawn up from the earth, or from the fountains, to the clouds by Iris, the messenger of Juno, that is of the air, Ovid. Met. i. 270.; and the goddess of the rainbow; hence Bibit ingens arcus, Virg. G. i. 380.*; that thus the clouds were fed, and rains produced, Lucan. iv. 79.; Ovid. ib.; Senec. 2, N. i. 6. Stat. Theb. ix. 405. to which opinion Plautus humorously alludes, Cure. i. 2. 41.

Clouds often appear very beautifully coloured, from their being compofed of aqueous particles, between which air is inter(perfed; and therefore they exhibit various colours, according to the different diftances of thefe aqueous particles.

The opinions of several of the ancients, particularly of Plato and Pythagoras, concerning the nature of light and of colours, came wonderfully near to this juftly-celebrated discovery of Newton, concerning the caufe of colours. They taught, that colours were the effect of light tranfmitted from bodies, and containing small particles adapted to the organ of vifion; Plu

So-Pelago folitam Thaumantida (i. e, Irida,) pasti, Stat. Achill. i.220.
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tarch.

tarch. de placit. philof. i. 15. iv. 13.; Qui (colores) quoniam quodam gignuntur luminis itu, Lucret, ii. 207. & 754. 794, &c. Plato has even described the effects of the mixture of colours, and fays that two colours might be formed out of one; and while he points out the difficulties of this fubject, and the merit of the perfon who fhould fully explain it, he, as it were by the fpirit of divination, pronounces the nobleft eulogium on Newton. Plato in Timao.

The doctrine of Defcartes concerning the inftantaneous propagation of light, was maintained by fome of the followers of Ariftotle, who even employed the fame fimilitude with him to illuftrate it; that, as a long ftick or firing, when stretched, cannot be moved at one end without being inftantly moved at the other, so if an impulfe be made on a particle of the fubtile fluid of light next the fun, it must be inftantaneously communicated to all the other particles between the fun and the organ of fight.

Ariftotle even explained the reason why the rays of the fun paffing through a small hole of a fquare or triangular shape, form a circular image; which is faid to have been first solved by Maurolicus or Marolle, about the middle of the 15th century. The reafon Ariftotle affigns is, that the rays converge into a cone, whose base is the luminous circle. Ariftotle, problem. 15.5.

In

The appearance of the Aurora Borealis, or Streamers, is fuppofed to proceed from electric matter in the atmosphere. ancient times it seems to have rarely happened, as it is scarcely ever mentioned by any of the Latin or Greek claffics. It was first observed in London in 1560, and called Burning Spurs. Since the year 1716, it has been more frequent. A history of it before that time was written by Halley. It is feldom feen in the fouthern hemisphere.

The ancients, although acquainted with the magnifying power of glafs, Senec. Nat. i. 3. feem to have had no inftruments of the optical kind, but fpeculums or looking-glaffes, and glass globes filled with water. These they are thought to have used in performing their minute works of art; and indeed we can scarcely conceive the poflibility of executing them without fuch affiftance. Thus Pliny mentions, on the authority of Cicero, the whole Iliad of Homer, written on parchment, in so fine a character, as to be contained in a nut-fhell, vii. 21. So Ælian speaks of an ivory chariot, fo fmall and fo delicately framed, that a fly with its wing could cover it; and a little ivory fhip of the fame dimenfious, i. 1. Pliny fays, that in his time artificers made

ufe of emeralds to affift their fight, which were made concave, the better to collect the rays (concavi ut vifum colligant); and that Nero used them in viewing the combats of gladiators, xxxvii. 5. f. 16. Seneca fays, that the smallest letters, which could hardly be difcerned with the naked eye, might be diftinctly read through a glafs tube filled with water; as the ftars appear larger when viewed through a cloud; Quæft. Nat. i. 6.; and Aulus Gellius fpeaks not only of multiplying mirrors, but also of such as made the objects appear inverted;

xvi. 18.

The magnifying and burning power of glaffes is fuppofed alfo to have been known to the Druids, from certain lenticular or fpherical gems of rock crystal, belonging to them, as it is thought, which are ftill preferved in the cabinets of the curious.

The doctrine of the refraction and reflection of light has been wonderfully illuftrated in modern times, by the various inventions and improvements of glaffes. Of these the most remarkable are the telescope and microscope, both refracting and reflecting.

The first telescope was made by Zachary Janfen, a maker of spectacles at Middleburgh, in 1590. Galileo, profeffor of mathematics at Padua, having heard of this difcovery in 1609, but without feeing any fuch inftrument, fet himself to contrive one of the fame kind; in which he fucceeded, and in a fhort time carried his improvements to a furprising degree of perfection: See p. 29. But it was Kepler who firft explained the teafons of the effects of telescopes.

The reflecting telescope was invented by Mr. James Gregory of Aberdeen, and improved by Sir Ifaac Newton. But the greatest improvement on telescopes of the dioptric kind, was made fome years ago by Mr. Dollond; who ingeniously contrived a method of correcting the defects of former inftruments of this kind, arifing from the different refrangibility of the rays of light, by the application of two different kinds of glass, poffeffed of different powers of refraction, and of different figures, so that the effects of the one exactly counterbalance thofe of the other.

In a reflecting telescope we never fee the object itself, but only that image of it which is formed next the eye in the teleícope.

The magnifying power of the reflecting telescope was confiderably increased by Mr. Short, and has been lately augmented to a wonderful degree by Dr. Herschel.

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The microscope was invented foon after the telescope by Janfen and his fon, improved chiefly by Leuwenhoek, a Dutch physician, who was born at Deift in 1632, and died 1723. The microscope has as much extended the fphere of human knowledge, with refpect to the nature and properties of minute fubftances, as the telescope in viewing distant objects. . ¿

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Of MOTION, and its LAWS...

MOTION is the removal of a body from one place to another, or a continual change of place.

Any force acting upon a body to move it is called a Power. The momentum or quantity of motion is in proportion to the force imprefied. The heavier any body is, the greater is the power required either to move it, or to top its motion. That fcience which teaches the effects of powers or moving forces, in as far as they are applied to engines, is called MECHANICS.

The fimple machines employed to move bodies are called mechanical powers. These are fix in number, the lever, the wheel and axle, the pulley, the inclined plane, the wedge, and the fcrew.

1. The LEVER, a ftraight bar of iron or other fubftance, fupported on a fulcrum or prop, made ufe of to raife weights to a fmall height, is of two kinds; 1. When the weight to be raifed is on one end, the force applied on the other, and the prop between both, but much nearer to the weight than the power, which is the common fort of lever.. 2. When the prop is at one end, the ftrength applied to the other, and the weight between them. Thus in raising a water plug in the streets, the workman puts an iron lever through the hole of the plug, till it reach the ground, and making that his prop, lifts the plug with his strength at the other end of the lever. Our common balance is a common lever; fo alio the Roman fteel-yard, (fatēra vel trutīna); the arms. (brachia) of which, or the two ends of the beam, are poifed on a prop, an axis or centre, whereon it turns. When the weights on each brathium are equal, the balance is faid to be in aquilibrio, or the weights to equiponderate. Unequal weights hanging at different distances from the centre may equiponderate.

2. A wheel turning together with its axis, hence called AXIS IN PERITROCHIO, which ferves to raise weights to a greater height. The power in this machine is applied to the circumference of the wheel, by the motion of which, a rope that

is tied to the weight, is wound about the axis, by which the weight is raised.

3. The PULLEY, or a little wheel moveable about its axis. with a rope running over it. A machine made by combining feveral pullies together, is often made use of to raise weights, when the axis in peritrochio cannot be applied.

4. The INCLINED PLANE, the advantage gained by which is as great as its length exceeds its perpendicular height. Thus if the plane be three times as long as high, a weight may be rolled up on it with a third part of the power which would be requifite to draw up the fame weight perpendicularly.

5. The WEDGE; which may be confidered as two equally inclined planes joined together.

6. The SCREW; which cannot properly be called a fimple power, because it is never used without the application of a lever, or winch, to affift in turning it; and then it becomes a compound engine of very great force, either for preffing the parts of bodies, or for raising great weights.

Various machines are conftructed, in which these fimple mechanic powers are all combined; hence called compound machines. We may judge of the knowledge of the ancients in mechanics from the ftupendous works which they reared; especially from their moveable towers, and other warlike engines.

A thing of the greatest importance in mechanics is to discover the centre of gravity of bodies.

The CENTRE OF GRAVITY in a body is a point where all the parts of the body, in whatever pofition it is, are in æquilibrio. When the centre of gravity is fuftained, the body remains at reft. When feveral bodies are joined together, that point on which the whole may be poifed, is the centre of gravity. Henee a building will stand, although confiderably bent from the perpendicular, while its centre of gravity is fupported: as the famous tower of Pifa, which inclines feventeen feet; fo at Bologna, &c. Whatever point in a body or machine sustains the centre of gravity, fuftains the whole weight; fo that the force with which any body tends towards the earth, is, as it were, collected in that centre.

The laws of motion, established by Sir Ifaac Newton, which he calls the Laws of Nature, are three in number.

1. All bodies continue in the state they are in, whether of reft or motion, till they are obliged to alter that state by fome force impreffed. All motion is naturally rectilineal or in a straight line.