79 o THE POPULAR SCIENCE MONTHLY. 



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 Abbe Boulliau (Bullialdus) actually announced* 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 ; f 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 pro- 

 posal of the planetary revolutions as a problem in mechanics is due to 

 him ; and it has been immemorially held that prudens quwstio est 

 dimidium scientiw. 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 mo- 

 tions ; and possibly the prosecuting this speculation may give us a true hypothe- 

 sis 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. J 



On this matter, at least, Hooke's ideas were persistent and pro- 

 gressive. 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 pow- 

 er 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 effec- 

 tual 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 tbe undertaker, that he will find all the great motions of the world to 

 be influenced by this principle, aud that the true understanding thereof will be 

 the true perfection of astronomy. 



Our readers will perceive that he was at this tiie still at fault as, 

 to the rate of decrease of the central force ; but, some years later, this 



* "Astronomia Philolaica," Paris, 1645. 

 f " Theoricae Mediceorum Planetarium," Florence, 1666. 

 % Birch, "The History of the Royal Society," vol. ii., p. 91. 

 " An Attempt to prove the Motion of the Earth," p. 28. 



