relative to Black, Watt, and Cavendish. 481 



may bear down with it the bodies it pervades with force pro- 

 portional to all their parts it acts upon, nature making a cir- 

 culation by the slow ascent of so much matter out of the 

 bowels of the earth in an aerial form, which for a time consti- 

 tutes the atmosphere, but being continually buoyed up by the 

 new air, exhalations, and vapours rising under, at length 

 (some part of the vapours which return in rain excepted) 

 vanishes again into the aetherial spaces, and there perhaps in 

 time relents and is attenuated into its first principles. For 

 nature is a perpetual circulatory worker, generating fluids out 

 of solids, and solids out of fluids, fixed things out of volatile, 

 and volatile out of fluid, subtile out of gross, and gross out of 

 subtile, some things to ascend and make the upper terrestrial 

 juices, rivers, and the atmosphere, and by consequence others 

 to descend for a requital to the former. And as the earth, 

 so perhaps may the sun imbibe this spirit copiously, to con- 

 serve his shining, and keep the planets from receding further 

 from him : and they that will may also suppose that this spirit 

 affords, or carries with it, the solary fuel and material principle 

 of light, and that the vast aetherial spaces between us and the 

 stars are for a sufficient depository for this food of the sun and 

 planets*." 



How far in a geometrical and mechanical point of view a 

 supposition which presents to us the problem of an uniform 

 central loss of force in a sphere of " tenacious or springy " 

 fluid, urged by a constant pressure, and drawing down or im- 

 pelling the bodies that float in it with a force proportional to 

 the number of their ultimate particles, can have been contem- 

 plated as tending to satisfy the conditions of the law of gra- 

 vity, I leave to mathematicians to judge. This supposition 

 preceded the public announcement of the law by ten years; 

 but Newton has himself stated that he had deduced that law 

 from Kepler's some twenty years before he published itf. 



He soon, however, in a letter to Boyle in 1678, abandoned 

 this form of hypothesis for one in which he supposes the aether 

 no longer a gradually absorbed, centripetal, atmosphere, but 

 a stationary fluid, " which consists of parts, differing from one 

 another in subtilty by indefinite degrees/' so arranged by the 

 force with which the 'pores of matter repel the aetherial particles 

 in proportion to their magnitude, "that from the top of the air 

 to the surface of the earth, and again from the surface of the 

 earth to the centre thereof, the aether is insensibly finer and 

 finer ;" and in an aetherial atmosphere so constituted he holds 

 that bodies would be propelled towards each other by the as- 



* Registry Book of the Royal Society, vol. v. from 1675 to 1679, p. 68. 

 f Letter of Newton to Halley, 1686. 



