Carrurners.—Retardation of Earth's Rotation by Volcanic Action. 857 
that on the surface, say, at a*depth of about 800 miles, and it may be taken, 
for the purpose of illustration, to decrease uniformly towards the surface. 
If we assume the thickness of the crust to be 400 miles, its average elasticity 
would be that due to its average depth of 200 miles, or 500 degrees. 
Adopting this elasticity we can calculate by a simple proportion the heat 
which will be generated when the earth masses at A, Fig. 1, fallin. If a 
compression of 1—40th of a foot generates one unit of heat, how much will 
be generated with a compression of one-eighth of a foot when the resistance 
is increased 500 times? It would be 2500 of our units, each of which 
would warm a mile of stone one degree Fahrenheit. 
As the earth masses have only a quarter of a foot to fall, the whole 
action would occupy about a seventh of a second. The molecules require 
a sensible time to transmit motion from one to another, the rate of travel 
being probably not more than a mile a second ; the whole of the molecular 
motion would therefore be confined to a thin vein extending for only one- 
seventh of a mile on each side of the line of fracture, but reaching from the 
surface to the inner side of the solid crust. 
From this line of action the energy would spread along the crust, not by 
the slow process of the conduction of heat, but it would be transmitted at 
the rate of a mile a second. 
The difference between the conduction and transmission of heat may be 
illustrated by a modification of a time-honoured experiment. If the first 
of a row of suspended balls be warmer than the last, it will be a long time 
before any of the excess of heat reaches the latter by conduction, but if the 
first be lifted and allowed to fall, heat is generated, transmitted through the 
other balls, becomes potential energy, and may be converted into heat by 
allowing the last ball to fall again. 
It is by this rapid means that the energy collected along the line of 
fracture is spread ; it will find the readiest means of getting into equili- 
brium, which would be by expanding the matter forming the crust. If we 
assume that four-fifths of it take this form within 500 miles of the line of 
fracture, we should then have 2,000 units of heat (being four-fifths of the 
2,500 above mentioned) spread over five hundred miles. This would raise 
the average temperature of the whole by four degrees; the second earth 
mass at A, would also have its temperature raised in the same manner, 
giving together a mass 1,000 miles long and 500 miles thick. The expan- 
sion of such a mass when warmed four degrees would give a range of para- 
bolic-shaped hills 1,000 miles wide at the base and 860 feet high. 
As the earth masses were assumed to be 2000 miles long by one mile 
wide, or 1-10,000th part of the earth’s area, the whole volcanic energy 
represented by a decrease of three feet in the carth’s compression, or in 
