1902-3.] Mr G. Eomanes on Cause of Earth's Internal Heat. 417 
resulting from the density and force of gravity jointly at all points 
above it. An inspection of the curve showing the pressures 
resulting from this assumed regular increase of density shows that 
these increase in a faster ratio than the depth for about three- 
quarters of the distance to the centre, after which the increase 
goes on more and more slowly to the centre. If the increase in 
density were adjusted to be more nearly in proportion to the 
pressure, it would have to commence more slowly than it has been 
here supposed to do, and this would entail a higher density at the 
•centre, because the average density must come out 5 '5 27 in any 
case ; hence the arrangement chosen probably avoids exaggeration 
of the density and pressure at the centre. The following is the 
calculation of pressure at the centre : — 
20,900,000x 345 292 
2 X 153 
6880 x 10 6 lbs. per square foot. 
The first factor is half the radius of the earth in feet multiplied 
into the weight at the surface of a cubic foot of density 5* *527, and 
represents what the pressure in pounds per square foot would be 
if the density were uniform throughout, and the second factor is 
the ratio, found by graphic process, between what the assumed 
regular increase of density would cause the pressure to be and 
what uniform density would cause it to be. If, now, we assume 
that each cubic foot has been subjected to half * this final pressure 
as an average in compressing it from density 2*76 to density 
13*2, we shall probably again avoid exaggeration. This gives 
6880 x 10° 10*44 
13*2 
= 2720 x 10° foot-pounds, as the energy ex- 
pended in compressing and heating each cubic foot (i.e., each mass 
of 172*5 lbs.)f near the centre. 
The energy required to raise one cubic foot of water through 
lbs. Joule’s Equivalent. 
1° Tahr. is 62*5 x 772 = 48,250 foot-pounds, which is only about 
X6 , Ijoo 1 * the en ^rgy which, we have found, may have been 
* If we were dealing with a fluid this would be too high, but with rock the 
case is different, for the compression will lag very greatly behind the corre- 
sponding pressure, and the amount of lag will always be decreasing. 
t This is equivalent to 15,768,000 foot-pounds of energy expended on each 
pound of matter, which, again, is equivalent to a fall to the surface from over 
three times the earth’s radius above it. 
