[ 205 ] 
V. Addition to the Paper on “ Volcanic Energy : an attempt to develop its true Origin 
and Cosmical Relations”*. By Robert Mallet, A.M., C.E., F.R.S., M.B.I.A. 
Received April 3, — Head May 7 , 1874. 
In the paper whose title is given above (Philosophical Transactions, part i. 1873) the 
author has shown upon experimental data, and upon the acknowledged basis that the 
amount of heat annually dissipated from our globe equals that evolved by 777 cubic 
miles of ice at 32° melted to water at the same temperature, what is the amount of heat 
that can be annually produced by the transformation of the mechanical work of mean 
rock when crushed by the descent of the external shell upon the nucleus contracting 
beneath it; he has also estimated the annual supply of heat necessary for the main- 
tenance of the volcanic activity at present existing upon our globe ; has shown that its 
total amount cannot exceed a small fraction of the entire heat dissipated annually, being 
only Y 5 t 9 thereof, or, in terms of crushed mean rock, equal 0-5579 of a cubic mile (para- 
graphs 179 and 197); he has also given, in Table II. (page 201) and succeeding para- 
graphs, his experimental results as to the contraction by diminution of temperature of 
melted matter that may be presumed similar to the rocky material of our globe from 
which natural lavas are derived. This contraction in volume, in relation to temperature 
between that of the blast-furnace and of the atmosphere, is shown graphically by the 
curve Plate X. of the above paper, the upper and lower portions of the curve being 
derived from experiment. The preceding elements afford some of the data necessary 
for any calculation as to the actual contraction of our globe now taking place annually 
by its secular refrigeration ; but the author refrained from attempting any such calcula- 
tion on the grounds that other data indispensable to any certain result are yet wanting. 
If we knew the thickness of the earth’s solid shell and the true increment of hypogeal 
temperature from the surface to the centre, or even the mean temperature of the nucleus 
and the nature of the whole of the matter composing the latter, we might with some 
assurance approximate to the amount of annual contraction of the globe due to refri- 
geration. But of the deep interior of our planet we really Jcnow but two things, viz. 
that the interior is hotter than the exterior, and what is the mean density of the whole. 
By making certain suppositions, however, as to some of the chief data wanted, we may 
approximate to some probable measures of the present annual contraction, and be 
enabled to see how far the results tend to sustain or overthrow the views enunciated by 
the author as to the nature and origin of volcanic heat and energy, and may also find 
* Bead June 20, 1872 ; Philosophical Transactions for 1873, p. 147. 
2 F 
MDCCCLXXV. 
