AND ON THE REMOTE HISTORY OF THE EARTH. 
531 
As the lunar effects increase in importance (when we look backwards), the relative 
value of the solar effects decreases rapidly, because the solar tidal reaction leaves the 
earth’s orbit sensibly unaffected (see Section 19), and thus the solar effects remain nearly 
constant, whilst tire lunar effects have largely increased. The relative value of the 
several tidal effects is exhibited in Tables II. and III. 
Table IY. exhibits the length of day decreasing to a little more than a quarter of its 
present value, whilst the obliquity diminishes through 9°. But the length of the 
month is the element which changes to the most startling extent, for it actually falls 
to iVth of its primitive value. 
It is particularly important to notice that all the changes might have taken place in 
57 million years; and this is far within the time which physicists admit that the earth 
and moon may have existed. It is easy to find a great many verce cciusce for changes 
in the planetary system ; but it is in general correspondingly hard to show that they 
are competent to produce any marked effects, without exorbitant demands on the 
efficiency of the causes and on lapse of time. 
It is a question of great interest to geologists to determine whether any part of 
these changes could have taken place during geological history. It seems to me that 
this question must be decided by whether or not a globe, such as has been considered, 
could have afforded a solid surface for animal life, and whether it might present a 
superficial appearance such as we know it. These questions must, I think, be answered 
in the affirmative, for the following reasons. 
The coefficient of viscosity of the spheroid with which the previous solution deals is 
'lOCt 
given by the formula —— tan 35° (see Section 11, (40)), when gravitation units of force 
are used. This, when turned into numbers, shows that 2‘055xl0 7 grams weight 
are required to impart unit shear to a cubic centimeter block of the substance in 
24 hours, or 2,055 kilogs. per square centimeter acting tangentially on the upper 
face of a slab one centimeter thick for 24 hours, would displace the upper surface 
through a millimeter relatively to the lower, which is held fixed. In British units 
this becomes,—13-^ tons to the square inch, acting for 24 hours on a slab an inch thick, 
displaces the upper surface relatively to the lower through one-tenth of an inch. It 
is obvious that such a substance as this would be called a solid in ordinary parlance, 
and in the tidal problem this must be regarded as a rather small viscosity. 
It seems to me, then, that we have only got to postulate that the upper and cool 
surface of the earth presents such a difference from the interior that it yields with 
extreme slowness, if at all, to the weight of continents and mountains, to admit the 
possibility that the globe on which we live may be like that here treated of. If, 
therefore, astronomical facts should confirm the argument that the world has really 
gone through changes of the kind here investigated, I can see no adequate reason for 
assuming that the whole process was pre-geological. Under these circumstances it 
must be admitted that the obliquity to the ecliptic is now probably slowly decreasing; 
