270 Cllley — Fundamental Propositions in the 



Two tennis raquets are seemingly the same. But one, when 

 struck, emits a much higher tone than the other. It is strung 

 tighter than the other. 



A piece of wire has been bent repeatedly back and forth at 

 one point. It looks much the same there as at other points, 

 but we know it is much more ready to break there than else- 

 where. 



A sheet of brass has been run through between heavy rolls. 

 It looks like another sheet that has not had this experience, 

 but we know that it cannot be bent so sharply without break- 

 ing as the unrolled sheet. 



So we could proceed, giving many other and familiar 

 instances of things that look alike while actually in very dif- 

 ferent conditions. And if we consider in what the difference 

 in condition consists we shall find in each instance that there 

 was a straining of one part of the body against another, that 

 there existed a condition of initial or, as I prefer to call it, 

 u primary " strain. 



Of the existence of these primary strains frequently evi- 

 dence is only obtainable through the application of external 

 force until injury ensues, for elastic action up to this point is 

 usually practically unaffected by the presence of such strains. 

 Only occasionally, as in the case of the bicycle wheels and the 

 tennis raquets, is the contrary true and can non-injurious tests 

 disclose and define these strains. In this case acoustic tests 

 will perform this service ; with glass, polarized light answers 

 the same purpose. 



Among the more prominent causes of these primary strains 

 is sudden and unequal cooling from the molten state, the 

 Prince Rupert drop furnishing a startling example of this but 

 other cases being numerous, particularly among metal castings. 

 The lamp chimneys are partly examples of this but partly 

 examples of the consequences of sudden and unequal heating, 

 of which thick bottles and tumblers, broken by suddenly till- 

 ing them with hot water, are further examples. Then there 

 are the consequences of the unequal expansion and contraction 

 of different substances in contact. The breaking of the welded 

 rail was due to this, it being put under great strain by con- 

 tracting far more than the ground in which it lay. The same 

 thing occurs in masonry and iron construction where the iron 

 is firmly anchored to the masonry without any allowance for 

 expansion and contraction. Wood and iron when united cause 

 similar trouble, the swelling and shrinking of the wood from 

 varying moisture adding a further factor. Cement and con- 

 crete in setting also make trouble, and many other instances 

 could be found. And although this straining is usually objec- 

 tionable, in some cases it is put to good use as in the balance 



