ME. EOBEET MALLET ON VOLCANIC ENEEGY. 
219 
221. We also see that it is but a partial view to say the volcano is a safety-valve to 
the earthquake ; for the volcano is really the safety-valve for the relief from time to time 
of the effects of contraction by cooling of our globe ; and there is perhaps no more 
convincing consideration, indicating that the motive cause here assigned for vulcanicity 
is the true one, than is found in the fact that the assigned mechanism is one of nature’s 
balancing adjustments, that the volcano’s work is exactly proportionate to the crushing 
energy of the contraction that brings it forth, and which is thus drained off, gradually 
on the whole, though intermittently, in place of accumulating, until by the crushing 
together of vast masses at once of the earth’s thick crust, cataclysm must arise destructive 
to the living creation. The action resembles that of the escapement of a clock, which 
lets the weight drop not uniformly, but slowly and gently, which if permitted to 
descend suddenly through a large space, must destroy the machine. 
222. If, then, this be the true nature of volcanic activity upon our globe, it must be 
so for other planets, so far as their construction is analogous to that of our own. 
And should future improvements of the telescope ever enable us to examine the sur- 
faces of the other bodies of our solar system with sufficient exactitude, we shall be able 
in some degree to test this. At present, however, we can only apply it to our own 
satellite, and ask does it give us any information as to the peculiarities that its surface 
reveals to us in respect to bygone vulcanicity therein. 
223. Without ocean or atmosphere, volcanic action such as we have upon our globe is 
impossible ; it is possible, however, that the former vulcanicity of the moon was main- 
tained until her whole ocean (which, in that case, must have been a very limited one) and 
her atmosphere had been wholly absorbed. 
If that be so, as a precise balance is not probable, then it is likely there is still more 
or less unoxidized or otherwise chemically uncombined material in the moon. 
But whether the vulcanicity of the moon be a completely spent energy or not, we find 
upon its surface elevations, and what appear to be craters, vastly exceeding in altitude 
and in width any thing upon our globe, and which seem perfectly abnormal as compared 
with the relative small size of the moon to our earth. 
224. If vulcanicity be dependent merely upon communication, through the crust, with 
the liquid nucleus, partial or universal, and water-access to the same, there is no ima- 
ginable connexion between the intensity of volcanic action and the size of the planet 
on which it occurs ; or if any, it is this, that the smaller planet cooling fastest may have 
a thicker crust, and so volcanic intensity should show itself less the smaller the planet is. 
But if it be, as here contended, a consequence of secular cooling, then the intensity will be 
greater as the progress of cooling is more rapid. 
Now the cooling of any globe of like constitution and at the same distance from the 
D 2 
sun must be directly as its surface and inversely as its mass — that is to say, as 13 being 
the mean diameter. Hence the rate of refrigeration of the moon from this cause alone 
must have been greatly more rapid than that of our globe, and hence the intensity of vul- 
mdccclxxiii. 2 G- 
