1866. | Adams’ Recent Astronomical Discovery. 519 
the interplanetary spaces. It is also certain that any increment 
thus caused in the mass of either is followed by a decrement 
(inconceivably minute of course) of the major axis of the lunar 
orbit, and a consequent increase in the moon’s mean angular 
velocity around the earth. 
There remains only to be considered the question of a possible 
retardation of the earth’s motion of rotation. This solution of the 
difficulty was discussed by Laplace.* The only cause assigned in 
his time for such a retardation was the influence of the trade-winds 
upon the great mountain chains. He showed that this cause was 
insufficient to affect appreciably the earth’s period of rotation. 
And further, as his calculations appeared to afford a complete 
explanation of the lunar acceleration, he felt himself justified in 
pronouncing his famous dictum on the constancy of the sidereal 
day. Of course, now that this accordance between theory and 
observation has been shown not to exist, the question is reopened. 
A cause of retardation not considered by Laplace has also been 
suggested, Mayer having pointed out (in 1848) that the tides “do 
work” on the earth, and that this work is the equivalent of 
a certain amount of “vis viva” lost in the earth’s motion of rotation. 
M. Delaunay has now shown that this cause would probably be 
sufficient to account for the acceleration of 6” in a century. 
Assuming this explanation to be the true one, it appears that 
our great terrestrial time-piece, hitherto supposed to be keeping 
most perfect time, requires correcting and rating. In the course 
of the last 2,000 years it has lost nearly an hour and a quarter, 
and compared with its motion at the beginning of that interval, it 
is now losing one second in twelve weeks. The day is also 
lengthening, and will continue to do so until it is equal mm length 
to the lunar day, that is, to our lunar month. The inhabitants of 
earth may console themselves, however; for the rate of change will 
diminish continually, and even if it did not, thirty-six billions of 
years would have to elapse before the change would be fully 
effected. 
But although the races at present inhabiting the earth are not 
likely to be affected, either for good or evil, by the process of 
change we have been considering, it is impossible not to look with 
interest into the vista of the far future, and trace our earth in its 
gradual progress to the condition now presented by the moon,—to 
its degradation, may we not surmise, from the position it now holds 
as a life-sustamer. Looking backward into the far past, we see a 
progress of a like nature through which our moon has passed while 
the earth’s strong influence has been exerted on her rotation, 
* The explanation is suggested parenthetically by Newton in the second 
edition of the ‘Principia:’ ‘‘Halleius noster motum medium Lune cum motu 
diurno terre collatum paulatim accelerari primus omnium quod sciam deprehendit.” 
