16 Geological Effects of a Varying Rotation of the Earth. [ January, 
earth lost seventeen seconds in its rotation between 1750 and 
1800, and has gained thirty-one seconds since 1800 A.D.’ Also, 
that much greater variations may have taken place in the past, 
and yet escaped the observation of astronomers: 
Mr. G. H. Darwin, in his paper on the Precession of the Vis- 
cous Spheroid, and on the remote history of the earth, shows 
that if we assume a viscosity for the earth, such that a cubic inch, 
when subjected to a force of 1314 tons, would be distorted ġ of 
an inch in twenty-four hours, and that such viscosity has remained 
constant, the retardation of the rotation of the earth has been 
such that 46,300,000 years ago a sidereal day was fifteen hours 
thirty minutes, and a sidereal month 18.62 days. As Mr. Darwin 
remarks: “It seems that we have only to postulate that the up- 
per and cooler 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 possibil- 
ity that the globe on which we live may be like that here treated 
of.” Hence we may start with the assumption that not only is 
the rotation of the earth variable, but this variation is con- 
-siderable. 
It is proposed to present, in the treatment of our subject, first, 
a theoretical discussion of the case, secondly, a survey of related 
facts, and finally, suggestions to direct further investigation of the . 
matter. ne 
I. The rotation of the earth may be considered as the result of — 
gravitation condensing it from its original nebulous or gaseous — 
‘condition, The check to condensation we may consider to have — 
been originally the expansive power of heat and the centrifugal — 
component of its force of rotation. Now, however, we find the f 
equilibrium of rotation lying between the following forces : i 
Forces affecting the rotation of the Earth. Those tending 
to accelerate are: 
First. The contraction of the earth, especially in low latitudes. 
This, now, as in all past ages, is doubtless the main force on this 5 
side. 
Second. The transfer of matter of any kind, from lower to ~ 
higher latitudes. This may be (1) either by the transfer of liquid 5 
matter in the interior, attending a local depression of the earth's — 
crust in lower latitudes, or (2) a transfer of water, either as vapO% 
1 See also Am. Fourn. of Sci: (111), XIV, p: 408. 
? Phil. Trans., Vol. 170, Part 11, 
