a principle existing, but proved its existence, power, and prevalence. He did not divine the nature of the force, but clothed it with the certainty of mathematical demonstration, and showed its explanatory power in accounting for the visible occurrences in the universe.

The prince of philosophers, as Newton has been justly styled, was born in the year 1642, at Woolsthorpe in Lincolnshire. The scene of his birth, a farmhouse of homely appearance, situated in a small retired valley, has been carefully preserved in honour of his memory. In early life he attended to the business of the farm; but owing to his fondness for books, mechanical contrivances, and mathematics, his friends wisely determined upon preparing him for another vocation; and in his eighteenth year he was sent to Trinity College, Cambridge. His rapid progress in various branches of science soon indicated his future eminence; and at the age of twenty-seven he was appointed to the professorship of mathematics at the University. But before this period he was in possession of many beautiful discoveries, and had made great advances towards solving the problem of the celestial motions. Biographers relate, that having quitted Cambridge in 1666 to avoid the plague, Newton was sitting one day in the garden at Woolsthorpe, when he was led to reflect upon that mysterious power which causes all bodies near the earth to tend towards its centre. This power being apparently unimpaired at the greatest altitudes of the earth's surface, the tops of the highest buildings and the summits of the loftiest mountains, the question proposed itself "May not the same force extend its influence to a great distance from the earth, even to the moon? May it not be the very reason why the moon is continually drawn away

from the straight line in which projected bodies tend to move, and is thus made to circulate around the earth? And may not a similar principle determine the paths of the planets round the sun?"

After seventeen years of intellectual toil, Newton rigidly demonstrated the attraction of gravitation, of which terrestrial gravity is only an individual example, to be a universal law; and from the sublime height to which the discovery raised him, he looked out upon the grand phenomena of the universe clearly explained because certainly referable to its agency-the stone rolling down the mountain side, the tide swelling in the ocean, the moon wheeling round the earth, the planets revolving round the sun, the satellites circulating round their primaries, and the far-roving comets returning to pay their homage to the monarch of the system. The law may be expressed as follows:-All bodies in the universe, whether great or small, attract each other, with forces proportioned to their respective quantities of matter, and inversely as the squares of their distances from each other. This pervading law is remarkable for its simplicity and grandeur. It asserts, first, that every particle of matter attracts, and is attracted by, every other particle. We see no proof of this between small bodies at the terrestrial surface, because it is rendered insensible by the enormously greater mass of the earth which attracts them to its own centre. But falling bodies, whether a pebble from the hand, an apple from the tree, an avalanche from the Alps, or an aerolite from the sky, manifest the earth's attractive power. Secondly, the mutually attractive force of bodies varies according to their masses; and thirdly, its intensity depends upon their distance. It diminishes as the square of the distauce

increases. If the earth were removed to two, three, or four times its present distance from the sun, the solar attraction would be diminished as the square of these numbers, or be respectively four, nine, and sixteen times less, while its own attraction on the sun would be proportionately decreased. Or if the two bodies were brought in similar degrees nearer, this mutual attraction would be increased in the same ratio. It is said that Newton, towards the close of the investigation, foreseeing the result, became too excited to proceed with his computations, and was compelled to resign the task of completing them to a friend.

The principle of universal gravitation may now be viewed in connexion with the simple laws of motion, derived from a rigorous examination of those moving bodies which are subject to man's closer scrutiny, for the Divine wisdom has impressed them upon bodies celestial as well as terrestrial, as is evident from the certainty with which astronomical predictions are fulfilled. If a body, situated in space and free to move, receives an impulse capable of giving it a certain velocity, it will go forward in a straight line for ever in the direction of the impulse, and with uniform velocity, if no resisting or diverting force is encountered; and it will resume its straight-forward motion, whenever the force that turns it aside is withdrawn. But if a body is acted upon at the same time by two impulses in different directions, the resulting motion will be in a direction determined by their joint action. This condition applies to the case of a body revolving in an orbit, like the moon round the earth, and the planets round the sun; and is the determining cause of their orbital route. Thus the tangents to the circle, or the straight lines A. B. in the diagram may repre

A

sent the rectilinear path in which the earth or any planet would travel under the influence of the projectile force which launched it into universal space; while the straight lines A s define the different direction it has

a tendency to pursue owing to the attraction of the sun at s. If the solar attraction, or centripetal force, were suspended, the earth would immediately desert the circular route, and fly off at a tangent to it, in the same manner as the stone which the school boy whirls round his head in a sling flies off in a straight line the moment one of the strings is let go, as it is then set free from the force that binds it to the circle. On the other hand, if the projectile or centrifugal force were suspended, the earth would fall to the sun. But being perfectly surrendered to the joint influence of the two different forces, the earth is constantly deflected by their regulated strife from the straight lines A B and A s, and pursues a curvilinear path, as the result of their combined action. Thus without iron trackways to define their course, or bars and bolts to keep them in place, the planets are securely poised in space, move freely through it in vast orbits round the sun, and accomplish with unerring certainty their respective cycles. "The Lord by wisdom hath founded the earth; by understanding hath he established the heavens."

The demonstrations of Newton were published in the "Principia," in the year 1687, under the auspices of the Royal Society, but at his own expense. The name implies that the work contains the fundamental principles of natural philosophy. It is divided into

three books. The first treats of motion in free space; the second is chiefly occupied with questions relating to resisted motion; and the third is upon the system of the world, by which is to be understood, the arrangement of all the bodies that compose the material universe, with their relations to each other, now commonly denoted by the phrase, the "mechanism of the heavens." During the two years that the illustrious author was engaged in preparing his work, he seemed to live only to calculate and think, often acted unconsciously, took no cognizance of the ordinary concerns of life, and had to be reminded by others of the time of his meals. The volume, upon its first appearance, was read with admiration by the leading mathematicians of the day among his own countrymen; and its doctrines were soon taught at all the universities. On the continent, the theory of gravitation was generally rejected for at least half a century. But the hour of universal triumph came at last; and finely has the truth of its principles been illustrated by foreign geometers, while they have cheerfully recognised Newton as the Father of Physical Astronomy. Yet though one of the most eminent of them has assigned to the Principia the pre-eminence over all the other productions of the human intellect, its author, when receiving on one occasion the compliments of his friends, thus reviewed his career:-"I know not what the world will think of my labours, but to myself it seems to me that I have been but as a child playing upon the sea-shore, now finding some pebble rather more polished, and now some shell rather more agreeably variegated than another, while the immense ocean of truth extended itself unexplored before me."

At the advanced age of eighty-five, Newton died in