ADDRESS. 23 
‘slowly retarded by tidal friction in much the same way that a fly-wheel 
is gradually stopped by any external cause of friction. Since action and 
reaction are equal and opposite, the action of the satellites on the planet, 
which causes the tidal friction of which I speak, must correspond to a 
reaction of the planet on the motion of the satellites. 
At any moment of time we may regard the system composed of the 
rotating planet with its attendant satellite as a stable species of motion, 
but the friction of the tides introduces forces which produce a continuous, 
although slow, transformation in the configuration. It is, then, clearly of 
interest to trace backwards in time the changes produced by such a con- 
tinuously acting cause, and to determine the initial condition from which 
the system of planet and satellite must have beer slowly degrading. We 
may also look forward, and discover whither the transformation tends. 
Let us consider, then, the motion of the earth and moon revolving in 
company round the sun, on the supposition that the friction of the tides 
in the earth is the only effective cause of change. Weare, in fact, to 
discuss a working model of the system, analogous to those of which I 
have so often spoken before. 
This is not the time to attempt a complete exposition of the manner 
in which tidal friction gives rise to the action and reaction between 
planet and satellite, nor shall I discuss in detail the effects of various 
kinds which are produced by this cause. It must suffice to set forth the 
results in their main outlines, and, as in connection with the topic of 
evolution retrospect is perhaps of greater interest than prophecy, I shall 
begin with the consideration of the past. 
At the present time the moon, moving at a distance of 240,000 miles 
from the earth, completes her circuit in twenty-seven days. Since a day 
is the time of one rotation of the earth on its axis, the angular motion of 
the earth is twenty-seven times as rapid as that of the moon. 
Tidal friction acts as a brake on the earth, and therefore we look 
back in retrospect to times when the day was successively twenty-three, 
twenty-two, twenty-one of our present hours in length, and so on back- 
ward to still shorter days. But during all this time the reaction on the 
moon was at work, and it appears that its effect must have been such that 
the moon also revolved round the earth in a shorter period than it does 
now ; thus the month also was shorter in absolute time than it now 
is. These conclusions are absolutely certain, although the effects on the 
motions of the earth and of the moon are so gradual that they can only 
doubtfully be detected by the most refined astronomical measurements. 
We take the ‘day,’ regarding it as a period of variable length, to 
mean the time occupied by a single rotation of the earth on its axis ; and 
the ‘month,’ likewise variable in absolute length, to mean the time occu- 
pied by the moon in a single revolution round the earth. Then, although 
there are now twenty-seven days in a month, and although both day and 
month were shorter in the past, yet there is, so far, nothing to tell us 
whether there were more or less days in the month in the past. For if 
