ADDRESS. 25 
carries conviction with it, but I have necessarily been compelled to pass 
over various doubtful points. 
Time is wanting to consider other subjects worthy of notice which 
arise out of this problem, yet I wish to point out that the earth’s 
axis must once have been less tilted over with reference to the sun than 
it is now, so that the obliquity of the ecliptic receives at least a partial 
explanation. Again, the inclination of the moon’s orbit may be in great 
measure explained ; and, lastly, the moon must once have moved in a 
nearly circular path. The fact that tidal friction is competent to explain 
the eccentricity of an orbit has been applied in a manner to which I shall 
have occasion to return hereafter. 
In my paper on this subject I summed up the discussion in the follow- 
ing words, which I still see no reason to retract :— 
‘The argument reposes on the imperfect rigidity of solids, and on the 
internal friction of semi-solids and fluids ; these are vere cause. Thus 
changes of the kind here discussed must be going on, and must have gone 
on in the past. And for this history of the earth and moon to be true 
throughout it is only necessary to postulate a sufficient lapse of time, and’ 
that there is not enough matter diffused through space materially to resist 
the motions of the moon and earth in perhaps several hundred million years. 
‘It hardly seems too much to say that granting these two postulates 
and the existence of a primeval planet, such as that above described, then 
a system would necessarily be developed which would bear a strong 
resemblance to our own. 
‘A theory, reposing on vere cause, which brings into quantitative 
correlation the lengths of the present day and month, the obliquity of the 
ecliptic, and the inclination and eccentricity of the lunar orbit, must, I 
think, have strong claims to acceptance.’ ! 
We have pursued the changes into the past, and I will refer but 
shortly to the future. The day and month are both now lengthening, 
but the day changes more quickly than the month. Thus the two periods 
tend again to become equal to one another, and it appears that when that 
goal is reached both day and month will be as long as fifty-five of our 
present days. The earth will then always show the same face to the 
moon, just as it did in the remotest past. But there is a great contrast 
between the ultimate and initial conditions, for the ultimate stage, with 
day and month both equal to fifty-five of our present days, is one of great 
stability in contradistinction to the vanishing stability which we found in 
the initial stage. 
Since the relationship between the moon and earth is a mutual one, 
the earth may be regarded as a satellite of the moon, and if the moon 
rotated rapidly on her axis, as was probably once the case, the earth 
must at that time have produced tides in the moon. The mass of the 
moon is relatively small, and the tides produced by the earth would be 
! Phil. Traxs., pt. ii., 1880, p. &83. 
