26 REPORT—1905. 
large ; accordingly the moon would pass through the several stages of her 
history much more rapidly than the earth. Hence it is that the moon 
has already advanced to that condition which we foresee as the future 
fate of the earth, and now always shows to us the same face. 
If the earth and moon were the only bodies in existence, this ultimate 
stage, when the day and month were again identical in length, would be 
one of absolute stability, and therefore eternal ; but the presence of the 
sun introduces a cause for yet further changes. I do not, however, pro- 
pose to pursue the history to this yet remoter futurity, because our system 
must contain other seeds of decay which will probably bear fruit before 
these further transformations could take effect. 
If, as has been argued, tidal friction has played so important a part 
in the history of the earth and moon, it might be expected that the like 
should be true of the other planets and satellites, and of the planets them- 
selves in their relationship to the sun. But numerical examination of the 
several cases proves conclusively that this cannot have been the case. The 
relationship of the moon to the earth is in fact quite exceptional in the 
solar system, and we have still to rely on such theories as that of Laplace 
for the explanation of the main outlines of the solar system. 
I have as yet only barely mentioned the time occupied by the sequence 
of events sketched out in the various schemes of cosmogony, and the 
question of cosmical time is a thorny and controversial one. 
Our ideas are absolutely blank as to the time requisite for the evolu- 
tion according to Laplace’s nebular hypothesis. And again, if we adopt 
the meteoritic theory, no estimate can be formed of the time required even 
for an ideal sun, with its attendant planet Jove, to sweep up the wanderers 
in space. We do know, indeed, that there is a continuous gradation from 
stable to unstable orbits, so that some meteoric stones may make thousands 
or millions of revolutions before meeting their fate by collision. Accord- 
ingly, not only would a complete absorption of all the wanderers occupy 
an infinite time, but also the amount of the refuse of the solar system 
still remaining scattered in planetary space is unknown. And, indeed, 
it is certain that the process of clearance is still going on, for the earth is 
constantly meeting meteoric stones, which, penetrating the atmosphere, 
become luminous through the effects of the frictional resistance with 
which they meet. 
All we can assert of such theories is that they demand enormous 
intervals of time as estimated in years. 
The theory of tidal friction stands alone amongst these evolutionary 
speculations in that we can establish an exact but merely relative time- 
scale for every stage of the process. It is true that the value in years of 
the unit of time remains unknown, and it may be conjectured that the 
unit has varied to some extent as the physical condition of the earth has 
gradually changed. 
It is, however, possible to determine a period in years which must be 
shorter than that in which the whole history is comprised. If at every 
