518 Adams’ Recent Astronomical Discovery. [ Oct., 
her own breadth more in the course of any one century than in 
the course of the neat preceding century ! 
Adams himself offers no opinion as to the probable cause of the 
want of accordance between theory and observation which he has 
thus detected, beyond the remark that “if established the fact would 
be a most interesting one, and might put us on the traces of an 
important physical discovery.” 
It seems little likely that changes in any other element of the 
terrestrial orbit besides the eccentricity should (as has been sug- 
gested) appreciably affect the lunar acceleration. As to the direct 
action of the planets on the moon we have Huler’s opinion that no 
part of the secular inequality is due to this cause. But it may be 
worth while to examine this point afresh. 
Of the opinion offered by Euler that the acceleration is due to 
the resistance of an etherial fluid occupying space, Laplace in his 
earlier dealings with the subject expressed a not unfavourable opinion. 
But as it appeared that the same cause which produced the secular 
acceleration affected the motions of the moon’s apsidal and nodal 
lines, and. that the proportion existing between these variations cor- 
responded closely with observation, which would not be the case if 
a resisting medium accelerated the moon’s mean angular motion, he 
considered he had proved that no such medium exists. This proof 
is now set aside; and the possible effects of a resisting medium have 
still to be considered. It is hardly necessary to remark that if such 
a medium exists, the angular velocity of the planets must also undergo 
a slow process of acceleration. The argument in favour of a resist- 
ing medium, derived from the acceleration of Encke’s comet, will 
of course not be overlooked. 
Another solution, suggested and examined by Laplace, was that 
gravity acts progressively, not instantaneously. He showed that 
in order that such progressive transmission should cause the observed 
acceleration, its velocity must exceed the velocity of light eight 
millions of times. To explain that portion of the acceleration for 
which theory fails to account, the rapidity of transmission of gravity 
should be yet greater, should in fact be upwards of two thousand 
billions of miles per second! If, as seems probable, the difficulty 
can be explained without assuming the progressive transmission of 
gravity, it would follow of course either that the action of gravity 
is instantaneous, or that it is so greatly in excess of the last-named 
velocity that the effects of its progressive transmission are insensible. 
In the possible effects of increments accruing to the mass, 
either of the earth or moon, we have another possible solution of 
the acceleration, or of a part of it. It is certain that both the 
earth and moon as they sweep together round the sun, are con- 
tinually gathering fresh recruits from the bands of meteorites 
(pocket-planets, as Humboldt has termed them) which revolve in 
