Examination of the Theory of a Resisting Medium. 11 
ered for almost a quarter of a century afterwards. In 1786, how- 
ever, the true cause was revealed. In that year M. le Mar 
de Laplace discovered both the cause and the law of this accelera- 
tion. He demonstrated that it is produced by the action of the sun 
upon the moon ; that it varies with the eccentricity of the terrestrial 
orbit, and consequently that such acceleration is a necessary result 
of the law of universal gravitation.(41) In a chapter founded upon 
the assumed possibility of a resisting ethereal fluid, Laplace says: 
** Hence it follows, that the resistance of the ether can become sen- 
sible, in the moon’s mean motion only. Ancient and modern ob- 
servations evidently prove that the mean motions of the moon’s 
perigee and nodes are subject to very sensible secular inequalities. 
he secular motion of the perigee, deduced from the comparison 
of ancient and modern observations, is less by eight or nine sexa- 
gesimal minutes, than that which results from the comparison of the 
observations made in the last century. This phenomenon, of which 
no doubt can remain, must, therefore, depend upon some other 
cause than the resistance of ether. We have seen that it depends 
on the variation of the eccentricity of the earth’s orbit; and, as the 
secular equations resulting from that variation satisfy, completely, 
all the ancient and modern observations, we may conclude that the 
acceleration, produced by the resistance of an ethereal fluid, on the 
moon’s mean motion, is yet insensible.”(42) Again: “ the accord- 
_ance of theory with observation proves to us that if the mean move- 
ments of the moon are varied by causes foreign to the law of uni- 
versal gravity, their influence is so small as not yet to have become 
sensible.’’( 
The errour of Fracastor and Apian, in regard to the uniform di- 
rection of the tails of comets, has enjoyed an extent of credence 
not often secured to a false position. Although a direction nearly 
in prolongation of the radius vector of the sun and the comet is not 
unusual for these tails, yet observations very early furnished excep- 
tions enough to,destroy the rule which has been ‘ long adhered to 
in this particular. If, as Pingré supposed, the resistance of ether 
has any agency in producing these tails, we should always expect 
them to be situated behind the nucleus, relatively to the comet’s 
actual motion, without relation to the position of the sun: but this 
(41) Delambre, l’Astronomie au Dix-Huitiéme Siécle, p. 598. 
(42) Mécanique Céleste, (Bowditch,) vol. 3, p. 694. 
(43) Systéme du Monde, p. 229. 
