Address to the Royal Geological Society. 13 
might. do, in a short time, under the same small straining force. 
If the stick be supported at bcth ends and left so for several 
weeks, its own mere weight will be sufficient to make it bend 
downwards considerably in the middle. And yet while thus 
really a fluid it is all the time exceedingly hard, and if broken 
will snap with a conchoidal fracture, which is generally charac- 
teristic of hard and close substances. Now we seem to have 
good reason to believe that our globe, taken as a whole, is a 
viscous body in this sense; and therefore, though she be practi- 
cally as rigid as steel, relatively to the cycling, reciprocating, 
comparatively short-period tidal forces called the lunar semi- 
monthly declinational, and the monthly elliptic or parallactic, 
yet she may be very different indeed, relatively to the continued 
ever similarly directed decrement of the centrifugal force of 
rotation, which, relatively to the present matter, is equivalent to 
a positive deforming force. 
Let. us, then, see what reasons there are for believing our earth 
to be, as a whole, a viscous body. The explanation of viscosity 
given by Clerk Maxwell is, very briefly, somewhat as follows. 
Even in homogeneous and solid bodies, whose molecules have 
thermal agitation, the groups of molecules are not all similarly 
conditioned. The agitation of the molecules of particular groups 
may accumulate, so that ever and anon the configuration of a 
group will break up, and, if the body is under strain, take a new 
configuration, which will be adapted to the present relative po- 
sitions of the groups and free from strain. The tendency to 
do this depends partly on the amplitude of the heat oscillations, 
and partly on the amount of strain. If there are a suffi- 
cient number of groups disseminated through the body which 
are stable under the conditions, the substance will be a solid 
with a limited amount of viscosity, which all bodies have; and 
if all the groups or only the great majority of them can, one by 
one, behave as we have described, the body will be a viscous one. 
Let us now, without making others responsible, consider the 
probable conditions of a body at a far higher temperature than 
that of free uncompressed fusion, and yet kept solid by tremen- 
dous pressure. (Sir William still contemplates the concession to 
us of about 7000° F. as the temperature of the interior of the 
earth.) The amplitude of the oscillations of the molecules is now 
vastly greater than was possible under unenforced solidity. But 
