verse. Toward this consummation Gilbert contributed something by his theory of universal magnetism; and Galileo, as well as Bacon and Horrocks, foresaw that in this direction lay the coveted secret. In 1645 the Abbé Boulliau (Bullialdus) actually announced[1] that the force by which the sun holds the planets in their orbits must vary as the inverse square of their distance from him; in 1666 Borelli published at Florence some suggestive speculations on the subject;[2] in England, Wallis, Wren, and Halley, all eagerly scanned the question, and all arrived at close approximations to the truth. But it was undoubtedly Hooke whose arrow flew nearest to the mark. The first definite proposal of the planetary revolutions as a problem in mechanics is due to him; and it has been immemorially held that prudens quwstio est dimidium scientiæ. In a paper on "Gravity," presented by him to the Royal Society, March 21, 1666, the following noteworthy passage occurs:
If such a principle (central attraction) be supposed, all the phenomena of the planets seem possible to be explained by the common principle of mechanic motions; and possibly the prosecuting this speculation may give us a true hypothesis of their motion, and, from some few observations, their motions may be so far brought to a certainty that we may be able to calculate them to the greatest exactness and certainty that can be desired.[3]
On this matter, at least, Hooke's ideas were persistent and progressive. In 1674 he announced a forthcoming "system of the world, answering in all things to the common rules of mechanical motions," and founded on the three following suppositions:
1. That all celestial bodies whatsoever have an attraction or gravitating power towards their own centres, whereby they attract not only their own parts. . . but also all the other celestial bodies that are within the sphere of their activity. 2. That all bodies whatsoever that are put into a direct and simple motion, will so continue to move forward in a straight line till they are, by some other effectual powers, deflected and sent into a motion describing a circle, ellipsis, or some other more compounded curve line. 3. That these attractive powers are so much the more powerful in operating by how much the nearer the body wrought upon is to their own centres. Now, what these several degrees are, I have not yet experimentally verified, but it is a notion which, if fully prosecuted, as it ought to be, will mightily assist the astronomer to reduce all the celestial motions to a certain rule, which I doubt will never be done without it. But this I durst promise the undertaker, that he will find all the great motions of the world to be influenced by this principle, and that the true understanding thereof will be the true perfection of astronomy.[4]
Our readers will perceive that he was at this the still at fault as, to the rate of decrease of the central force; but, some years later, this
