THE CAUSES OF EARTHQUAKES 



had been used for the purpose of our experiment, and 

 had been packed to the brim with ointment or thick 

 liquid, and if it had been squeezed in a vice instead of 

 in our hands, then perhaps we should have provoked 

 still more striking symptoms of an earthquake. The 

 ointment might have broken out through the lid. 

 Perhaps even tiny jagged holes or craters would have 

 been formed in the lid. Thus we see how strain may 

 produce earthquakes. Take some more examples. 

 Suppose a cork is very tightly fixed in a wine bottle, 

 and in order to get it out we employ a very powerful 

 lever corkscrew. The neck of the bottle, under the effect 

 of the too powerful pressure put on the inside surface 

 of the glass, will crack or break. Similarly if we screw 

 down a microscope too hard on a slip of glass the 

 glass will often crack suddenly. Both these instances 

 recently occurred within the writer's experience, and 

 few readers can have escaped noticing one or other 

 of them. The breakage in these instances is always 

 caused because a strain is set up somewhere in the 

 glass there is more pressure at one point than another, 

 and the glass, unable to resist this unequal pressure, 

 gives way. 



What happens when it gives way? To answer this 

 question we had better carry our minds to the example 

 of the pill-box lid. If the top of the box were of 

 very brittle material, like pottery or glass, then after 

 the breakage we know that most likely one of the 

 broken pieces would be a little higher than the other 

 would perhaps overlap it. That is what we often see 



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