Examination of the Theory of a Resisting Medium. 9 
known purposes of the universe. ‘The opinions of Kepler, upon 
this subject, may not have received less credence, in the day they 
were uttered, than did his discovery of the fundamental laws of the 
celestial movements ; but they were promptly consigned to oblivion 
by the subsequent revelation that comets, no less than planets, be- 
long to our solar system, and move in ellipses more or less elonga- 
ted, about the sun, obeying the same laws as the grosser planets. 
Of Descartes’ system, and of its fate, we have spoken. That sys- 
tem was undermined by the discovery and.application of the law of 
universal gravitation ; and as this ether constituted all that was most 
essential to the Cartesian doctrine, the celestial motions were no 
sooner found to be carried on independently of its aid, than the 
whole theory was abandoned. Newton, himself, as we have seen, 
applied this substance, under the name of “a most subtile spirit,” 
to the production of certain results, in his Principles of Natural Phi- 
losophy, and again in his Opticks. The passages we have quoted. 
These positions appear to have had their origin in a desire so to ex- 
plain the doctrine of gravitation as to free it from the implied asser- 
tion that bodies act in places where they are not—a form of attack 
which the metaphysicians chose to employ against it. Yet this was 
but subjecting the question to new difficulties; as there is nothing 
like a satisfactory explanation of gravity in the existence of this elas- 
tick ether. True, a fluid disposed-as Newton has assumed, would 
urge bodies in the direction he supposed; but what could maintain 
this fluid in the condition of its density varying according to the as- 
sumed law, is as inexplicable as the gravity it was meant to explain. 
The nature of such a fluid, if unrestrained, must be to equalize the 
density of all its parts, to the destruction of this hypothesis.(35) 
That Newton did not consider gravity inherent in matter is manifest 
from the passages under consideration; and he most fully states this, 
in words, in one of his letters to Dr. Bentley, as quoted by Prof. 
Playfair. Yet how he should have supposed he had escaped its ne- 
cessity by his resort to the agency of this ether—since it is clearly 
for this purpose that he sought its aid—may well be deemed inexpli- 
cable. “If two particles of matter, at opposite extremities of the di- 
ameter of the earth, attract one another, this effect is just as little in- 
telligible, and the modus agendi is just as mysterious, on the sup- 
position that the whole globe of the earth is interposed, as on that 
of nothing, whatever, being interposed, or of a complete vacuum 
(35) Playfair ot Math. end Pye 8 Science , pt. 2, sec. 4, 
Vout. XXXIII.—No. 1 Q 
