Newton, the supreme generalizer . . . Faraday, the master of experiment . . . Bell, the inventor of the telephone, transmits speech by a beam of light. Having now taken a rapid general view of observation and experiment, of the faculty of sound theorizing, let us enter the presence of two great masters of research and invention, beginning with a man who united the loftiest powers as a mathematician, a physicist, and a generalizer. How Newton Discovered the Law of Gravitation. How Sir Isaac Newton discovered the law of gravitation is thus told in his Life by Sir David Brewster:--“It was either in 1665 or 1666 that Newton’s mind was first directed to the subject of gravity. He appears to have left Cambridge some time before August 8, 1665, when the college was dismissed on account of the plague, and it was, therefore, in the autumn of that year, and not in that of 1666, that the apple is said to have fallen from the tree at Woolsthorpe, and suggested to Newton the idea of gravity. When sitting alone in the garden, and speculating on the power of gravity, it occurred to him that, as the same power by which the apple fell to the ground was not sensibly diminished at the greatest distance from the centre of the earth to which we can reach, neither at the summits of the loftiest spires, nor on the tops of the highest mountains, it might extend to the moon and retain her in her orbit, in the same manner as it bends into a curve the path of a stone or a cannon ball, when projected in a straight line from the surface of the earth. If the moon was thus kept in her orbit by gravitation, or, in other words, its attraction, it was equally probable, he thought, that the planets were kept in their orbits by gravitating towards the sun. Kepler had discovered the great law of the planetary motions, that the squares of their periodic times were as the cubes of their distances from the sun, and hence Newton drew the important conclusion that the force of gravity, or attraction, by which the planets were retained in their orbits, varied as the square of their distances from the sun. Knowing the force of gravity at the earth’s surface, he was, therefore, led to compare it with the force exhibited in the actual motion of the moon, in a circular orbit; but having assumed that the distance of the moon from the earth was equal to sixty of the earth’s semi-diameters, he found that the force by which the moon was drawn from its rectilinear path in a second of time was only 13.9 feet, whereas at the surface of the earth it was 16.1 in a second. This great discrepancy between his theory and what he then considered to be the fact, induced him to abandon the subject, and pursue other studies with which he had been occupied.