ASTRONOMY.
THE time was now come when the world was to be enlightened by a new science, arising out of the comparison of the phenomena of motion as observed in the heavens, with the laws of motion as known on the earth. Physical astronomy was the result of this comparison, a science embracing greater objects, and destined for a higher flight than any other branch of natural knowledge. It is unnecessary to observe, that it was . by Newton that the comparison just referred to was instituted, and the riches of the new science unfolded to mankind.
This young philosopher, already signalized by great discoveries, had scarcely reached the age of twenty-four, when a great public calamity forced him into the si tuation where the first step in the new science is said to have been suggested ; and that, by some of those common appearances in which an ordinary man sees nothing to draw his attention, nor even the man of genius, except at those moments of in-. spiration when the mind sees farthest into the intellectual world, In 1666, the plague forced him to retire from Cambridge into the country; and, as he sat one day alone, in a garden, musing on the nature of the mysterious force by which the phenome na at the earth's surface are so much regulated, be observed the apples falling spontaneously from the trees, and the thought occurred to him, since gravity is a tendency not confined to bodies on the very surface of the earth, but since it reaches to the tops of trees, to the tops of the highest buildings, nay, to the summits of the most lofty mountains, without its intensity or direction suffering any sensible change, Why may it not reach to a much greater distance, and even to the moon it self? And, if so, may not the moon be retained in her orbit by gravity, and forced to describe a curve like a projectile at the surface of the earth Here another consideration very naturally occurred. Though gravity be not sen sibly weakened at the small distances from the surface, to which our experiments ex .
tend, it may be weakened at greater distances, and at the moon may be greatly di minished. To estimate the quantity of this diminution Newton appears to have reason ed thus : If the moon be retained in her orbit by her gravitation to the earth, it is pro bable that the planets are, in like manner, carried round the sun by a power of the same kind with gravity, directed to the centre of that luminary. He proceeded,
therefore, to inquire by what law the tendency, or gravitation of the planets to the sun must diminish, in order that, describing, as they do, orbits nearly circular round the sun, their times of revolution and their distances may have the relation to one another which they are known to have from observation, or from the third law of Kepler.
This was an investigation which, to most even of the philosophers and mathemati cians of that age, would have proved an insurmountable obstacle to their farther pro gress ; but Newton was too familiar with the geometry of evanescent or infinitely small quantities, not to discover very soon, that the law now referred to would require the force of gravity to diminish exactly as the 'square of the distance increased. The moon, therefore, being distant from the earth about sixty semidiameters of the earth, the force of gravity at that distance must be reduced to the 3600th part of what it is at the earth's surface. Was the deflection of the moon then from the tangent of her orbit, in a second of time, just the 3600th part of the distance which a heavy body falls in a second at the surface of the earth ? This was a question that could be precisely answered, supposing the moon's distance known not merely in semidiameters of the earth but in feet, and her angular velocity, or the time of her revolution in her orbit, to be also known.
In this calculation, however, being at a distance from books, he took the common estimation of the earth's circumference that was in use before the measurement of Norwood, or of the French Academicians, according to which, a degree is held equal to 60 English miles. This being in reality a very erroneous supposition, the result. of the calculation did not represent the force as adequate to the supposed effect : whence Newton concluded that some other cause than gravity must act on the moon, and on that account he laid aside, for the time, all farther speculation on the subject. It was in the true spirit of philosophy that he so readily gave up an hypothesis, in which he could not but feel some interest, the moment he found it at variance with observation. He was sensible that nothing but the exact coincidence of the things compared could establish the conclusion he meant to deduce, or authorize him to proceed with the su perstructure, for which it was to serve as the foundation.