ME. EOBEBT MALLET ON VOLCANIC ENEEGY. 
209 
gravities would bring the mean density of the whole to 5 -5. The exterior portions of 
the sphere, constituting by far the largest portion of its entire volume, have a density of 
little more than 2-0. But we cannot deal with the absolutely unknown, nor assign 
either specific heat or specific gravity to the extremely dense material, whether metallic 
or not, which we must suppose to exist about the centre of figure of our planet. The 
most reasonable supposition, therefore, that we can make in reference to our present 
object is to neglect the nature of this extremely dense matter, and to assume the whole 
nucleus as composed of material not greatly different from the hardest and densest rocks 
with which we are acquainted, and, with some allowance for their further increase in 
density by compression, to adopt fof the whole nucleus a value for a density of 2 -75 
(or one half the mean density of our entire globe), and for its specific heat s'=0’200, 
being a little above the mean experimentally ascertained by the author for the five 
hardest and densest rocks in Table I. column 27 of his paper in Philosophical Trans- 
actions, 1873. The equation 
C 
s' X §' 
therefore becomes 
8265-4 
0-2x2-75 x 62-425 
=24°-74 Fahr., 
which is the amount of refrigeration produced by a unit in volume (1 cubic foot) of 
melted ice upon an equal volume of the nucleus. Having for the constant refrigerative 
power the 777 cubic miles of melted ice, and having the volume of the nucleus for any 
assigned thickness of shell, we at once obtain the amount of refrigeration of the nucleus ; 
and applying to that the partial mean coefficient of contraction for 1° Fahr. found at 
the upper portions of our curve, we are enabled to calculate the reduction in volume, 
and hence the diminution in radius, due to the amount of heat abstracted in the unit of 
time, viz. one year. The author has assumed four successive thicknesses for the shell, 
viz. 
1001 
200 i miles, 
400 f 
800 j 
and proceeding on the above principles has calculated the total annual contraction of 
the nucleus for each case. The partial mean coefficient of contraction adopted for that 
of the nucleus has been the mean between the two highest partial means shown in the 
curve and Table I. above given, viz. 0-0000769 for 1° Fahr. 
The final results obtained are comprised in Table II., before referring to which, 
however, some explanation and reference to diagram fig. 2 are necessary. 
E being the radius of our globe=3957 - 5 English miles, 
r=the radius assumed for the nucleus, whose thickness =11— r. 
Let the nucleus be assumed to contract by loss of its heat transmitted through the 
