Examination of the Theory of a Resisting Medium. 5 
the existence of this fluid was still retained by Newton, who sought 
to employ it ina new capacity. ‘And now we might add some- 
thing concerning a certain most subtile spirit, which pervades and 
lies hid in all gross bodies; by the force and action of which spirit, 
the particles of bodies mutually attract one another at near distances, 
and cohere if contiguous; and electrick bodies operate to greater dis- 
tances, as well repelling as attracting the neighbouring corpuscles ; 
and light is emitted, reflected, refracted, inflected, and 
ies.”(12) He furthermore supposed that this substance is spread 
through all the heavens; and when for lack of demonstration, un- 
certainty arose in his mind, he thus queried: “Is not this medium 
much rarer within the dense bodies of the sun, stars, planets and 
comets, than in the empty celestial spaces between them? And in 
passing from them to great distances, doth it not grow denser and 
denser perpetually, and thereby cause the gravity of those great 
bodies towards one another, and of their parts towards the bodies ; 
every body endeavouring to go from the denser parts of the medium 
towards the rarer?”’(13) 
In 1762 the Academy of Sciences, of Paris, proposed, for a prize, 
the question, ‘‘ Do the planets revolve in a medium of which the 
resistance produces a sensible effect wpon their movements?” For 
this prize M. Pabbé Bossut was the successful competitor. His cal- 
culations showed him that the effect of resistance, offered to the 
planets, would be to diminish the axis of their orbits, and conse- 
quently to shorten their periods of revolution. An acceleration in 
the movements of the moon had been observed, which was without 
explanation, and on applying his reasonings to the motions of this 
planet he satisfied himself that the observed acceleration was due to 
the resistance of ether, encountered by the moon, in traversing her 
orbit. The sum of that resistance he measured; and this theory 
being equally applicable to all the planets, he extended it to them 
all, and subjected each to the resisting influence of the ether.(14) 
The tails of comets were objects of early attention; and it was 
remarked, both by Fracastor and Apian, that the tail of the comet 
(12) Newton’s ere Principles of Natural Philosophy, vol. 2, p. 393. 
The copy of Newton to which all references in this article are made is the Eng- 
lish translation, eh Andrew Motte, 12 mo. edition, in two vols. London, 1729. 
(13) Newton’s Opticks, third edition, London, 1721, p. 325. 
(14) Bailly, Histoire de P’Astronomie Moderne, tome 3, p. 237; and Bossut, His- 
toire des Mathématiques, tome 2, p. 409. 
