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Yet the simile of an ocean billow is hardly correct, for sea waves 
do not really advance (except indeed the disastrous earthquake 
water waves), they move simply up and down in closed circles, as 
you may see a rope do when held at one end. An earthquake is 
like an air or sound wave, made, as you know, by the alternate 
compression and expansion of its particles. The upward or vertical 
shock is most felt just above the centre of disturbance—the 
** seismic centre” as it is called (seismos being Greek for earth- 
quake), and this centre it is possible to discover by carefully ex- 
amining the lines of fracture in different buildings, and following 
the horizontal line of vibration until it intersects an upright line. 
I will make this quite plain by a diagram on the black board. 
This seismic centre is thus found to be situated at a depth below 
the surface of from 5 to 80 miles, a distance quite insignificant 
when we consider that the depth from the surface to the centre 
of the earth is 4,000 miles. If I draw a circle as large as this 
board will hold to represent the world, the thinnest line I can make 
will represent a crust of over 50 miles in depth. As to what is the 
actual condition of the centre of our globe, there is great uncertainty, 
and much difference of opinion exists among eminent men of 
science. When the theory of the condensation of our earth, first 
from a nebulous or gaseous, and then from a fluid condition was 
first accepted, it was almost universally considered that the centre 
was still in a fluid and superheated condition, while the thin outer 
erust was hard and rigid, just as a circular mass of molten iron 
would cool and harden first on the outside while the centre was still 
fluid and hot. But there are astronomical facts which negative this 
idea. Professor Hopkins has demonstrated, at any rate to his own 
satisfaction, that the crust of the earth must have a thickness of at 
least 800 to 1000 miles, and that the whole globe must possess a 
rigidity equal to a mass of steel of the same size; and the balance 
of scientific opinion is now decidedly in favour of a solid central 
nucleus of a ferruginous character. One thing however is quite 
certain that the temperature increases in direct proportion to the 
depth below the surface, at the rate of one degree Fahrenheit for 
every 55 or 60ft., so that at a depth of 200 miles or so, we should 
_ reach a temperature of 18,000 deg., ‘‘ the effective temperature ”’ of 
_ the sun, and sufficiently high to liquify and volatilize every known 
earthly substance, metals, rocks, or crystals. But we must bear in 
mind that the boiling point of water is much higher under pressure, 
and that the same rule holds good as to the melting point of metals 
or rocks, and the superincumbent pressure they must be subject to 
in the centre of the earth must be something almost inconceivable, 
and probably sufficient to keep matter solid at any temperature. 
After all we must remember that we are arguing from very slender 
