20 Examination of the Theory of a Resisting Medium. 
served to us; nor are we better circumstanced, in reality, with re- 
gard to the basis of the faith of Alhazen, Tycho Brahe, Kepler, 
&c., in regard to this subject: but this belief we must not forget, 
was not coupled, so far as we have seen, with the theory of re- 
sistance. 
2d. That when the Cartesian thetie © arose, this ether, being an 
indispensable agent thereof, was every where believed in; not, in- 
deed, as a resisting medium, but as a propelling one, which carried 
the planets forward in their orbits: this faith came to the ground 
with the doctrine of which it formed a part. 
3d. When the laws of universal gravitation had exposed the errours 
of the Cartesian system, we find Newton still vaguely imagining of 
and concerning this substance, but in language so indistinct as not 
always to be definable; at one time supposing it to be the cause of 
gravity, and at other times, by its unequal density, mechanically 
giving direction to the motions of the heavenly bodies: the errour of 
these views is apparent. 
Ath. The ingenious arguments of Bossut, which took the prize of 
the French Academy, in 1762, were supposed to have well shown 
the resisting agency of this ether, in the acceleration of the moon’s 
mean motion ; and no doubts of the truth of this arose, until Laplace 
demonstrated that such acceleration is wholly due to the law of uni- 
versal gravitation. 
5th. That this ether has offered to the movements of comets a 
resistance which has rendered its agency appreciable. If the ob- 
jections that have been offered against this are valid, they are much 
more than sufficient to destroy even its plausibility. 
If the conclusions at which we have arrived, then, be correct, we 
have shown that the existence of this ethereal medium was for a 
long series of years believed in, without evidence known to us; that 
it has been, during another long series of years, even to the present 
day, accredited, also, upon different points of evidence, at different 
periods of time, but all which evidence has failed to sustain the fact 
of its existence; and that, therefore, to be hereafter adhered to, 
fresh evidences of its truth will be requisite to render it more than 
a mere hypothesis, or gratuitous assumption: not that its existence 
has been disproved; but only that confirmatory evidence of that 
existence no longer remains. _ 
The predictions, therefore, that have pointed at the destruction of 
the solar system, through the agency of a resisting ether, may very 
