212 
ME. EOBEET MALLET ON VOLCANIC ENEEGY. 
energy, and also to support the views of those who regard the solid crust of our globe as 
necessarily much thicker than geologists generally have been in the habit of admitting 
it. It is probable that the contractions here determined for our planet are below the 
truth; for 
1st. Some contraction must always take place through cooling of the solid shell itself, 
and especially of its lowermost and hottest portions, which has been here neglected. 
2nd. It is probable that the coefficient of contraction employed is below the truth 
for the material of the nucleus such as we have supposed it. 
If the central parts of the nucleus be metallic, it is probable that their coefficient of con- 
traction may largely exceed that here employed, while their specific heat is considerably 
less than that adopted for the entire nucleus. On the other hand, it must be remembered 
that a wave of heat from the central parts of the nucleus may take ages to travel con- 
ductively outwards to the lower surface of the shell, even when the latter is assumed 
800 miles in thickness, which is one of the reasons why in what precedes these central 
parts have been supposed of a nature similar to the nucleus. It follows that, on the 
supposition of a shell of 800 miles in thickness, the annual diminution in diameter of 
our globe, due to its secular refrigeration, may somewhat exceed, but cannot be less than, 
1493 3 96 9 9 64 1 4 diameter, a mere film wholly incapable of being recognized by the 
senses ; or taking the diminution of diameter from the unit of a British inch instead of 
a mile, it would amount in a period of 5000 years to a diminution of the diameter of our 
globe of 6*71616 inches, or less than 7 inches, a quantity so small that it must have 
escaped the most refined observation of the astronomer during the last 2000 years, even 
were we to suppose that during the whole of that period the instrumental resources of 
the astronomer were as perfect as at the present day. When we add to this the consi- 
deration that the matter composing the imaginary spherical shell of less than 3^ inches 
in thickness, which measures the contraction in volume of our globe during 5000 years, 
has by its refrigeration increased in density in the ratio at least of 1000 to 933, we readily 
discern the reasons for the negative results arrived at by Laplace in considering this 
question from the point of view of an observable diminution in the length of the day. 
Yet insignificant when thus measured as is the amount of annual contraction of our 
globe by its secular refrigeration, we see how important and mighty are its effects in 
preserving through the volcano the cosmical regimen of our world ; it is another added 
to the many instances already known in the range of natural philosophy, in which causes 
so minute a§ for long to remain occult to us are yet, though unseen and unnoticed, 
essential parts of the mighty machine. 
Three quantities related to each other indeed, but yet entirely different, have been 
treated of in the author’s present paper or in that of 1873. 
1st. The volume of mean rock which must be crushed annually in the earth’s shell 
in order to supply the heat necessary for existing annual vulcanicity, viz. (K5579 of a cubic 
mile, the heat due to which is fsV 9 of the total annually dissipated from our globe. 
