26. SAINT ANNE.

She was the mother of the Virgin Mary, and the wife of Joachim her father. Her festival is cele..

brated by the Latin church.

History of Astronomy.

[Continued from p. 174.]

Astronomy of Modern Europe.

HAVING, in the preceding sheets, shown, in as short a compass as possible, by what successive efforts the human mind attained to the knowledge of the celestial motions, it remains now to consider the means by which it has arrived at the general principle on which these laws depend. Des Cartes was the first who endeavoured to reduce the motions of the heavenly bodies to some mechanical principle. He, however, was very far from the truth: he imagined vortexes of subtle matter, in the centre of which he placed these bodies. The vortex of the Sun, forced the planets into motion; those of the planets, in the same manner, forced their satellites to move about them. The motion of comets, traversing the heavens in all directions, destroyed these vortices, as they had before destroyed the solid chrysalline spheres of the antient astronomers. Thus Des Cartes was not more successful in his mechanical than Ptolemy had been in his astronomical theory. But their la bours have not been useless to science. Ptolemy has transmitted to us the few astronomical truths which the antients had discovered. Des Cartes, by substituting, in the place of antient errors, others more seducing, and resting on the authority of geometrical discoveries, was enabled to destroy the empire of Aristotle and Ptolemy, which have stood the attack of a more careful philosopher; but by establishing, as a principle, that we should begin by doubting of every thing, he himself warned his con

temporaries, and those who came after him, to examine his own system with severity, which was soon found incapable of standing the test of rigid inquiry. It was reserved for the immortal NEWTON to teach the general principles of the heavenly motions, and to demonstrate the foundation on which they rested. This great man was born at Woolsthorpe, in the county of Lincoln, on Christmas-day, Old Style, 1642, the same year in which Galileo died, and who was, till he was twelve years of age, educated at two common day-schools, after which he was sent to a large school at Grantham, then under the care of Mr. Stokes. Here he gave proofs of a surprising genius, and astonished his schoolfellows by his mechanical contrivances. Instead of amusing himself, like other boys, with the sports familiar to his age, he was making curiosities and models of different kinds in wood. For this purpose he provided himself with all kinds of instruments on a small scale. Among his productions were a clock and a windmill, the latter in imitation of one lately set up near the town in which he was. These objects occasionally engrossed so much of his time as led him to neglect his books, but he could at any time recover his place, which he might lose by apparent negligence. After he had received a certain portion of school education, his mother called him home in order to assist in the business of a farm; but finding him not at all inclined to the duties of that station, she sent him again to Grantham school, and from thence, in a few months, to Trinity College, Cambridge, where he was admitted on the 5th of June, 1660. He surpassed every body: he endeavoured to inform himself on the subject which his tutor meant to lecture on; and when he came into the room the pupil usually knew more of the subject than himself. It is said, that a desire to know if they contained any thing in judicial astrology, first put him upon studying mathematics: in the elements of geometry he

did not find sufficient difficulty to engage his attention, and he threw it aside almost as soon as he had looked into it, a circumstance which he afterwards regretted. The geometry of Des Cartes, and the arithmetic of infinites by Dr. Wallis, engaged his studies, and on these he wrote comments as he proceeded.

He laid the foundation of all his discoveries before he was twenty-four years of age. In the year 1665, when retired to his own estate on account of the plague which had driven him from the University, the idea of the doctrine of gravitation first occurred to him, in consequence of seeing an apple fall from a tree. At that time, not being in possession of any accurate measure of the magnitude of the Earth, he estimated the force of gravity erroneously, and found that his theory was incapable of the purposes for which it was intended. He accordingly abandoned his hypothesis at that time, as erroneous: but, afterwards, when Picard had measured a degree of the Earth's surface, with tolerable accuracy, he was enabled to make a more precise estimate, and found that the force of gravity exactly accounted for the Moon's motion in her orbit. He applied his doctrine to the planets and the whole solar system, and soon found it to account, in a satisfactory manner, for the whole phenomena of the motions of these bodies.

In the year 1667, he was elected fellow of Trinity College, in Cambridge; and, in 1669, Dr. Barrow resigned his mathematical professorship to him. In 1671 he was elected a fellow of the Royal Society; and, in 1675, he had a dispensation from Charles II for retaining his fellowship without taking orders. In 1686 he published his Principia. In 1696 he was appointed Warden of the Mint, and in this office he did signal service in the great re-coinage which took place soon after. In 1699 he was made R

Master and Worker of the Mint, in which situation he continued till his death. In 1701 he appointed Mr. Whiston his deputy professor of mathematics at Cambridge, and gave him all the salary from that time, though he did not absolutely resign his professorship till two years afterwards. At this latter period he was chosen president of the Royal Society, and continued to fill that honourable situation till the time of his death. In 1705 he was knighted by Queen Anne, at one of her visitations to Čambridge. He was twice chosen Member of Parliament to represent the University of Cambridge. During the whole of his life he obtained the most distinguished honours of which his rank in life admitted; and the nation, to whose glory he had so much contributed, decreed him, at his death, public funeral honours.

We have observed that, when retired into the country, by the fall of an apple he had been led to establish the principle of gravity: he immediately carried his theory to solve all the phenomena connected with the solar system, and he discovered not only that the Moon was retained in her orbit by gravity, but the planets also were preserved in their orbits by their gravity towards the Sun; and he demonstrated this by the law that the areas described are proportional to the times. Now, it results from the relation of the squares of the times to the cubes of the greater axis of their orbits, that their centrifugal force, and, consequently, their tendency to the Sun, diminishes inversely as the squares of the distances increase from this body. Hence Sir I. N. found that bodies, in their fall, describe ellipses, of which the centre of the Earth occupies one of their foci; and then, considering that the planetary orbits are likewise ellipses, having the Sun in one of their foci, he had the satisfaction to see that the solution which he had undertaken from curiosity could be applied to the greatest objects in

nature. He has demonstrated that this relation exists in elliptic orbits generally, and that it indicates an equal gravity of the planets towards the Sun, supposing them at an equal distance from its centre. The same principle of gravity towards the principal planet exists likewise in all the systems of satellites, and Newton verified it on terrestrial bodies by very accurate experiments.

This great geometrician demonstrated, likewise, that a projectile can move in any conic section whatever, in consequence of a force directed towards its centre, and varying reciprocally as the square of the distances. He investigated the different properties of motion in this species of curves; he determined the conditions requisite for the section to be a circle, an ellipse, a parabola, or an hyperbola, which conditions depend entirely on the velocity and primitive position of the body.

These investigations, being applied to the motion of comets, informed him that they also move round the Sun, according to the same laws as the planets, with the difference only of their ellipses being very excentric; and he gave the means of determining, by observation, the elements of these ellipses.

. He obtained, likewise, from the comparison of the distance and duration of the satellites with those of the planets, the respective densities and masses of the Sun, and of the planets accompanied by satellites, and the intensity of the force of gravity at their surface.

By considering that the satellites move round their planets very nearly as if the planets were immoveable, he discovered that all bodies obey the same force of gravity towards the Sun. He was fully aware, that, by the equality of action and re-action, the Sun gravitated towards the planets, and these towards their satellites, and that there is an attraction. between the Earth and all the bodies that rest upon