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wroiaght iron, and at 100 miles from the surface all will be fluid ; 

 " the rocks will be as water." Hence it would appear that our 

 Globe, of 8,000 miles in diameter, is only a liquid mass contained 

 within a thin shell of 100 miles in thickness. That is to say, a 

 globe of 10 feet in diameter would have a crust of only one-eighth 

 of an inch in thickness. Hence it has been said that the varying 

 pressure of the atmosphere may be sufficient by weighing un- 

 equally on this thin shell to cause an oscillation of the crust, and 

 so give rise to earthquakes ; and, in proof of this • hypothesis,' it 

 has been adduced, that an earthquake is always preceded by a 

 fall in the barometer. Calculations have been made, to show 

 that the alteration in the atmospheric pressure indicated by two 

 inches lise or fall in the barometer, represents a pressure sufficient 

 to account for this, viz., 8 tons on the square yard, or 25 million 

 tons on the square mile. But when we think, that an increase of 

 pressure as shown by a rise of three inches, amounts to an increase 

 of weight on the crust of the earth equal to only one two hundred 

 and ninety-five thousandth part of its own weight (supposing it 

 to be only 100 miles thick), this would seem inadequate to pro- 

 duce any effect, on even so frail a bridge as this. 



This central heat theory is said to be a question particularly 

 interesting to Bathonians, as, according to tradition, we are to be 

 one day the victims of a want of balance in nature, and to sink 

 down into these subterranean fires. But I think we may re-assure 

 ourselves. In the first place, the increase of heat in proportion 

 to depth has been too hastily assumed, as a law, for it is clear 

 that local circumstances have a great effect in at least modifying 

 this increase ; at Wheal Clifford the increase of heat being 1° in 

 22 feet of descent, whereas at the mine of Dolbrouth 75 feet of 

 descent is requisite to give a similar increase, and in some mines 

 even 90 feet is requisite, while at the Lago di Monte Rotondo at 

 60 feet depth a temperature of 212° is reached. Theory would 

 say that the difierance would depend on the conductibility of heat 

 of the various rocks, and would be in an inverse ratio in propor- 

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