But the moft diftinguifhed aftronomer at this time was CHRISTIAN HUYGENS, Lord of Zuilichem, fon to the fe cretary of the Prince of Orange, born at the Hague 14th April 1629, and educated at Leyden under Schooten, the commentator of Defcartes-famous for the application of pendulums to clocks, and of springs to watches, for the improvement of telescopes and microfcopes, and for the great difcoveries which, in confequence of thefe improvements, he made in aftronomy.

The establishment of academies or focieties at this time contributed greatly to the advancement of science.

The Royal Society at London was firft begun in 1659, but did not affume a regular form till 1662. Its transactions were first published 1665.

The Academy of Sciences at Paris was founded in 1666, by Lewis XIV. who invited to it Roemer from Denmark, Huygens from Holland, and Caffini from Italy.

CASSINI was born at Perinaldo, in the county of Nice, 8th June 1625; came into France in 1669, and was appointed first profeffor in the royal obfervatory at Paris, where he profecuted his difcoveries till his death 1712; and was fucceeded by his fon. He was affifted by Picard, Auzout, and la Hire.

By the direction of the academy of fciences at Paris, a voyage was undertaken by Richer and Meuriffe, at the King's expence, to the island Caïenne, in South America, almoft under the equator, a. 1672, to afcertain feveral philofophical facts; the refraction of light, the parallax of Mars and of the Sun, the diftance of the tropics, the variation in the motion of pendulums, &c.

The parallax of the fun is the angle under which an observer at the fun would fee the femidiameter of the earth. This Caffini fixed at nine feconds and a half; and the angle under which we see the fun, at fixteen minutes and fix feconds or 966 feconds; hence he concluded, that these femidiameters, are as 9 to 966, or as 19 to 1932. So that, according to Caffini, the femidiameter of the earth is 100 times less than that of the fun; and confequently the fun is a million of times bigger than the earth.

The parallax of the fun has fince, from the tranfit of Venus 6th June 1761, and 3d June 1769, been discovered to be but about eight feconds; and confequently his comparative bulk to that of the earth, and his distance from it, to be proportionally greater. This method of finding the distance of the earth from the fun, and confequently of the other planets,

was

was firft propofed by Dr Halley; who had never feen, and was morally certain he should never fee this appearance.

Meuriffe died in the voyage, Richer returned in 1673. His anfwer on the parallax of Mars was not fatisfactory. Caffini calculated it at fifteen feconds.

The distance of the tropics was found to be 46° 57' 4".

But the chief advantage of this voyage was afcertaining the variation of the pendulum. In 1669, Placard remarked that clocks with a pendulum went flower in fummer, and fafter in winter; owing, as it was afterwards difcovered, to this, that heat dilates bodies, and confequently lengthens the pendulum; but cold contracts them, and therefore fhortens the pendulum. Richer found that the pendulum of a clock made 148 vibrations less at Caïenne than at Paris; that is, went 2 minutes 28 feconds a-day flower. Hence to adjust it, he was obliged to fhorten the pendulum.

The fame thing was confirmed by Halley while at St Helena, in 1676. But the motion of the pendulum is supposed to be alfo retarded near the equator, by its gravity being diminifhed on account of its greater distance from the centre of the earth, than near the poles.

About this time the French Jefuit miffionaries having got admiffion into China, contributed to the improvement of aftronomy. Father Schaal, one of their number, on account of his merit, and particularly of his skill in this fcience, was fo highly honoured at the court of China, that the Emperor, upon his death-bed, named him preceptor to his fon and fucceffor Can-hi. Schaal reformed the Chinese calendar, a matter of great importance in that country. It was ftill farther improved by Verbieft, who fucceeded Schaal about the year 1670. The moft eminent aftronomers in England, during this period, were Flamstead, Halley, and Hook.

FLAMSTEAD was born at Derby, 19th Auguft 1646. He compofed a new catalogue of the fixed ftars, containing about 3000. He made his obfervations first in private, and afterwards in the royal obfervatory of Greenwich, founded in 1675. He died in 1719.

These

HALLEY was born at London, 8th November 1656. He co-operated with Flamstead, in compofing the catalogue of ftars. In 1676, he was fent to St Helena to take a catalogue of the fixed ftars which do not rise above our horizon. be formed into conftellations, and to one of them gave the name of the Royal oak, in memory of that tree in which Charles II. faved himself from his purfuers. Halley was the first who

made an accurate obfervation of the tranfit of Mercury over the disk of the fun, which had been obfcurely feen by Gassendi in 1631, by Huygens and Hevelius in 1661. In trying to calculate from this obfervation the parallax of the fun, he perceived that it would be more exactly afcertained by the transit of Venus, which he knew would not happen before 1761. He, however, pointed out a method for this purpofe, which aftronomers have found of the greatest advantage. Under King

William, Halley was fent on feveral voyages, to obferve the variations of the compafs, and for other fcientifical purpofes. He fucceeded Flamstead in th. Royal obfervatory in 1719 and died 1742.

HOOK invented feveral aftronomical inftruments. He was of great fervice to Mr Boyle in completing the invention of the air-pump. Being appointed one of the furveyors for rebuilding London, he acquired a large fortune. He published feveral curious experiments which he had made to explain the motion of the earth and planets, on the principles afterwards adopted by Newton. He died in 1702.

Sir ISAAC NEWTON was born at Woolftrope, in the county of Lincoln, 25th December 1642, and ftudied at Cambridge. The rapidity of his progrefs in mathematical knowledge was aftonifhing. He perceived the theorems and problems of Euclid, as it were by intuition. At the age of twenty-four, he had laid the foundation of his moft important difcoveries. He was the first who gave a rational and complete account of the laws which regulate the motion of the planets, on the principles of the attraction of gravitation, now almost univerfally adopted. He is faid to have been firft led into his fpeculations on gravitation, as he fat alone in a garden, by obferving fome apples fall to the ground. Newton was ás remarkable for his modefty, as for the fuperiority of his genius. It was with difficulty he was prevailed on by the folicitations of Halley to publish his PRINCIPIA, or, Mathematical Principles of Natural Philofophy, in 1686, a work which was confidered as the production of a fuperior intelligence. In 1704, he publifhed his Optics; in 1711, his Fluxions, a new mode of arithmetical calculation, of great ufe in the higher parts of mathematics, the invention of which is difputed with him by Leibnitz, a German; and in 1728, his Chronology, which he endeavoured to adjust by calculating the periods of eclipfes. He received in his lifetime the honour due to his fingular merit. In 1703, he was elected prefident of the Royal Society; in 1705, he was knighted by Queen Anne. He was

twice member of parliament. In 1669, he was made master of the mint, which place, together with the prefidency of the Royal Society, he held till his death in 1726. His funeral was celebrated with great magnificence; the Chancellor and five other peers fupported the pall. He was buried in Westminfter Abbey, where a monument is erected to his memory.

The principles of Newton were illustrated and confirmed in France, by Giraldi, Caffini, Hire, Delifle, Louville, M. de la Lande, Maupertuis, Fontenelle, Mairan, M. de la Caille, &c.

-In Britain, by Flamstead, Halley, Whifton, Gregory, Defaguilliers, Molyneux, Bradley, Keil, Mercator, Mitchell, Long, Maclaurin, &c.

PRINCIPLES of the NEWTONIAN PHILOSOPHY, occafionally compared with the Opinions of the Ancients.

S Sir Ifaac Newton made all his difcoveries for explaining the motions of the planets, by reafoning from experiments or known facts, it will be requifite that the learner know the general principles on which he proceeded. These chiefly refpect the properties of matter, and the laws of motion.

I. GENERAL PROPERTIES of MATTER.

THE inherent properties of matter, or of body in general, are folidity, inactivity, mobility, and divifibility.

I. SOLIDITY and EXTENSION.-All matter has length, breadth, and thickness; hence every body is comprehended under fome shape or figure, and hinders all other bodies from Occupying the fame part of space that it poffeffes, which is called impenetrability. If a piece of wood be placed between two plates of metal, it never can be fqueezed fo hard, as to allow the plates to come into contact; and a small quantity even of water or air, if fixed between two bodies, can by no force be fo compressed, as to permit the bodies to meet one another, till the water or air be removed. Thus, if a globe of the hardest metal, with a hollow in the middle full of water,

be ftrongly compreffed, the water will ooze through its pores, and appear on the furface.

Space void of matter is called a vacuum, the existence of which, in oppofition to Defcartes, Newton maintained.

2. INACTIVITY, paffivenefs, or the vis inertia, i. e. the want of power in body to move itself. -Every body endeavours to continue in the state it is in, whether of reft or motion. Bodies on this earth, when fet in motion, foon ftop from the action of gravitation, from the refiftance of the air, or of friction. But if a body were carried to a certain diftance from the earth, and there projected in a particular direction, and with a certain velocity, it would continue for ever to move round the earth, without falling to it; as is the case with the moon.

3. MOBILITY, or the property in body, that it may be mcved from one place to another,

4. DIVISIBILITY ad infinitum, or without end, that is, no part of matter can be conceived fo fmall but there may still be a fmaller. Certain bodies may be divided into very minute parts. A grain of gold may be beaten into a leaf 50 inches fquare, which may be divided into 500,000 parts, visible to the naked eye if viewed with a microscope, that magnifies the object only ten times, the fifty millionth part of a grain may be fuppofed vifible. Mr Reaumur computes, that a grain of gold may be extended on a filver wire upwards of half a mile in length; and cover a furface of 1400 fquare inchies; fo that the thickness of the gold will be no more than the fourteen millionth part of an inch, that is, about 1200 times the thinness of ordinary gold leaf, which gold leaf is about 39 times thinner than thin poft paper.

But this is nothing to the fubtilty of parts in odoriferous bodies, and to the minutenefs of certain microfcopic animals, and their parts.

Similar notions concerning the infinite divifibility of matter were entertained by fome of the ancients; Cic. Acad. i. 7.; Plutarch. de placit. phil. i. 16. This property, however, exifts only in idea; for infinity in minutenefs, as well as magnitude, is altogether beyond our conception.

A body not eafily pierced or broken, or whofe parts cannot be eafily divided, is faid to be hard; the contrary, foft.

A folid body, eafy to be broken in pieces, is faid to be brittle; that which may be bent, pulled, or twisted, without breaking, is to faid be tough.

A body whofe parts yield to any impreffion, and are easily