In the year Galileo died (1642), Newton was born. At the age of twenty-four, when a plague was ravaging the cities of England, he secluded himself at his parental home in Lincolnshire and there contemplated the motions of the heavenly bodies. This work of his was put aside for two decades; it was not until the year 1686 that the first edition of Philosophiae Naturalis Principia Mathematica was published. A testimony is preserved that says the figures Newton had of the size of the earth and thus of the terrestrial radius were rather inexact—and consequently his computations of the Earth’s gravitational pull did not agree with observations. And, it is said, when the French savant, J. Picard upon measuring the meridian in Lapland, came to the correct result, that Newton became confident of his formula for inertia and gravitation.(1)

Life—claims (Hooke, Flamsteed, Leibnitz). Light corp.; space empty; how does gravitation act? nature of gravitation; law of simplicity.

When explaining his theory of celestial mechanics, Newton used the following example. A projectile—a stone—is thrown horizontally from the top of a high mountain; because of its weight it is

forced out of the rectilinear path, which by the initial projection alone it should have pursued, and made to describe a curved line in the air; and through that crooked way is at last brought down to the ground; and the greater the velocity is with which it is projected, the farther it goes before it falls to the earth. We may therefore suppose the velocity to be increased, that it would describe an arc of 1, 2, 5, 10, 100, 1000 miles before it arrived at the earth till at last, exceeding the limits of the earth, it should pass into space without touching it.(2)

At a very definite curve, the result of a very precise and definite velocity of projection, the stone would follow the circumference of the earth and “return to the mountain from which it was projected” without falling to the ground or flying off into space.

For the sake of this example, “let us suppose that there is no air about the earth or at least that it is endowed with little or no power of resisting,” and that only the weight of the projectile causes it to bend its path.

One can observe that these two figures differ by seven percent, and that therefore complete correspondence is an exaggeration. There are other, much more close correspondences involving our moon, and they still belong in the domain of coincidences. The moon, for instance, is placed so on its orbit that it appears nearly the same size as the sun, and actually, during the full eclipses, the moon chances so to cover the sun that only the solar corona is seen over the dark zone of the moon. Also the already mentioned fact that the moon’s mean distance is very nearly equal to sixty terrestrial semi-diameters, the number of seconds in a minute; or the fact that light travels 186,000 miles in a second, and the diameter off.

At the age of fifty, when the biological process of involution generally sets in in man, Newton became ill and depressed. The excessive exploitation of his brain, his unrelenting search for answers to nature’s unsolved problems undermined and disturbed the mental balance of the genius. When Newton was forty-five years old, his Principia was published. Then he worked on optics. The story goes that he left his manuscript on the table close to a burning candle and went out of the room to look at a procession; a pet overturned the candle and the manuscript burned. This misfortune started his depression. It is questionable whether this is a true story. In a young man mental depression usually sets in when the person faces a big task and is afraid to fail; in the second half of life, the person becomes depressed mostly as the result of slighting and humiliation. It would be wrong to think that a person who is great is protected by his greatness from the feeling of slighting and humiliation. Newton’s experiences with Hooke, with Leibnitz, and with Flamsteed could have been the real cause.

Edlestone, Correspondence, p. LXIII.

Brewster, Life of Newton, II, 142.

Dr. Ferd. Rosenberger, Isaac Newton und seine physikalischen Prinzipei (Leipzig, 1895).

Letter to Pepys (p. 278 in Rosenberger)

It is possible and even probable that if Newton lived in our time he would not support his theory of the mechanical movement of the planets. At the end of the Principia he wrote:

But hitherto I have not been able to discover the cause of those properties of gravity from phenomena, and I have no hypotheses; for whatever is not deduced from the phenomena is to be called an hypothesis; and hypotheses, whether metaphysical or physical, whether of occult qualities of mechanical, have no place in experimental philosophy.

Thus he felt that he left his theory of gravitation unjustified because he was unable to explain the cause of gravity and the nature of this phenomenon. However, he must have had some intuitive inkling of where to look for explaining gravitation, because on the same page, which is at the end of the third book of Principia, he wrote:


  1. But cf. the comments of F. Cajori in his edition of Newton’s Principia (Berkeley, 1946), p. 663. Cf. also the Mathematical Gazette 14 (1929), p. 415.

  2. Isaac Newton, The System of the World, Sec. 3, published with his Principia , transl. Motte, ed. F. Cajori (1946).