THE BUILDING OF THE EARTH 



breakages in the earth's crust, or the earth's interior, 

 must be very great indeed, or else that the earth must 

 be composed of rather shaky materials. Well, perhaps 

 both these suppositions are true. We spoke just now 

 of the instruments which seismologists use to record 

 earthquakes. They are known as "seismometers,"" and 

 a great many of them are used in Japan and on the 

 Californian or Pacific coast of America. Now it is 

 perhaps scarcely necessary to say here (when we recollect 

 how many cyclones and anticyclones England receives 

 from the Atlantic) that a storm or rainy weather is 

 usually heralded or accompanied by a fall in the 

 barometer, or a depression. Now when there is a 

 depression in the barometer that means that the weight 

 of air above the barometer is less than it was before, 

 though it is not so great a difference that human beings 

 could tell it, unless it were accompanied by other signs. 

 But the earth can tell it, and the mere fall of the 

 barometer, owing to changes of the air, will make the 

 earth tremble or quiver slightly, as if it were a jelly. 

 We cannot perceive it ; but the delicate seismometers 

 can ; and when a storm is coming to Japan or to 

 California from the Pacific, the instruments show that 

 the earth feels the passage of it. The comparison of the 

 earth to a jelly a very stiff jelly is on the whole a 

 useful one. 



If a very tall jelly is allowed to stand for some time, or 

 if the table on which it stands is shaken a good deal, 

 then, as we know, rifts will sometimes appear in the jelly. 

 The reason for these breakdowns in the jelly's composition 



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