164 Proceedings of the Asiatic Socicty. [ Ave. 
acceleration of the Moon’s motion. It has been suggested that the 
ether supposed to fill space, and whose existence had been con- 
sidered probable from the peculiarities of the motion of Encke’s 
comet produced a similar though less result on the moon, but De- 
launay suggested that the tides should produce a retardation of 
the Earth’s velocity which might account for it. 
Since then the theoretical discussion of the result of the tidal 
action has been in question. Col. Tennant had not been in the way 
of seeing all the solutions, but the problem is very intricate even on 
the limited hypotheses which render it analytically possible. The 
Astronomer Royal has shown that on the supposition of a canal 
equatorially surrounding the earth, the result of using the first 
order of small quantities only in the solution is that the friction 
produces no effect on the velocity of revolution, but that it retards 
the moon. This is precisely the opposite of the result sought, but 
he has also found in the terms of the second order one which 
would retard the earth. 
This or any thing similar is very far from giving a trustworthy 
value of the retardation, however, and he (Col. T.) would caution 
all to hesitate before giving faith to figures on this subject. 
Mr. Ayrton replied :— 
With reference to Colonel Tennant’s remark that ‘ Inertia is 
the passive power by which change of state is resisted, and that 
when a body is at rest there is no question of time involved,” 
it will be obvious that the first half of this sentence related to 
two things, a body at rest and a body in motion, whereas the 
second half of the sentence refers only to a body at rest. Now 
as regards a body in motion, inertia is the property it possesses to 
move uniformly. What, I ask, is meant by moving uniformly if no 
reference is to be made to time? Also what is meant by equal 
times, if no reference is to be made to the inertia of a body in mo- 
tion ? I therefore maintain, as before, that the two ideas inertia and 
equal times are so intimately connected, that neither can be ex- 
plained without reference to the other. | 
What follows in Colonel Tennant’s remarks rather tends to show 
the difficulty of applying direct calculation to the actual motion of 
