336 EARTHQUAKES. 



earthquake, as for instance in Derbyshire, may have been 

 produced by pulsations disturbing the equilibrium of 

 ground in a critical state. 



The falling in of subterranean excavations is also 

 possibly connected with these phenomena. 



Possible causes of earth pulsations. — Mr. Greorge 

 Darwin, in a report to the British Association (1882), has 

 shown that movements of considerable magnitude may 

 occur in the earth's crust in consequence of fluctuations 

 in barometrical pressure. (A rise of the barometer over 

 an area is equivalent to loading that area with a weight, 

 in consequence of which it is depressed. When the 

 barometer falls, the load is removed from the area, which, 

 in virtue of its elasticity, rises to its original position. 

 This fall and rise of the ground completes a single 

 pulsation.) 



On the assumption that the earth has a rigidity like 

 steel, Mr. Darwin calculates that if the barometer rises 

 an inch over an area like Australia, the load is sufi&cient 

 to sink that continent two or three inches. 



The tides which twice a day load our shores cause 

 the land to rise and fall in a similar manner. On the 

 shores of the Atlantic, Mr. Darwin has calculated that 

 this rise and fall of the land may be as much as 5 inches. 

 By these risings and fallings of the land the inclination 

 of the surface is so altered that the stile of a plummet 

 suspended from a rigid support ought not always to hang 

 over the same spot. There would be a deflection of the 

 vertical. 



In short, calculations respecting the effects of loads 

 of various descriptions, which we know are by natural 

 operations continually being placed upon and removed 

 from the surface of various areas of the earth's surface, 

 indicate that slow pulsatory movements of the earth's 



