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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 30 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 

 crust 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 sufiicient to keep matter solid at any temperature. 

 After all we must remember that we are arguing from very slender 



