FATIGUE OF METALS. 689 



tricky. As soon as adjacent regions in the air are at different pressures, we have 

 phenomena which reveal the existence of air. 



Bodies all tend to preserve the same temperature and also the same electrical 

 potential. Any disturbances in electrical equilibrium are much more quickly 

 obliterated than in case of thermal equilibrium, and we therefore see less of 

 electrical phenomena than of thermal. In thunder storms we see such disturb- 

 ances, and with delicate instruments we find them going on continuously. 

 Changes in temperature occurring on a large scale in our atmosphere, occurring 

 in these gas jets, in our fires, in the axles of machinery and in thousands of other 

 places, are so familiar that we have ceased to wonder at them. 



If we rub these two bodies together, the potential of the two is no longer 

 the same. We do not know which one has become greater, and in this respect 

 our knowledge of electricity is less complete than of heat. We assume that the 

 gutta percha has become negative. If we now leave these bodies in contact the 

 potential of the cat's skin will diminish and that of the gutta percha will increase 

 until they have again reached a common potential — that of the earth. As in the 

 case of heat and cold, we may say either that this has come about by a flow of 

 positive electricity from the cat's-skin to the gutta percha, or by a flow of nega- 

 tive electricity in the opposite direction, for these statements are identical. 



In case of our gas cylinders, the gas tends to leak out of the vessel where 

 the pressure is great, into the vessel where it is small. The heat tends to leak 

 out of a body of high temperature into the colder one, or the cold tends to go in 

 the opposite direction. Similarly, the plus electricity tends to flow from the body 

 having a high potential to the body having a low potential, or, the minus elec- 

 tricity tends to go in the opposite direction. 



FATIGUE OF METALS. 



For fourteen years State Geologist CoUett, of Indiana, has been experiment- 

 ing upon a theory that the best of iron, when subjected to continuous strain, 

 would undergo changes in its structure which would, after a time, render its use 

 dangerous, and that these structural changes were the explanation of many other- 

 wise inexplicably accidents, particularly to railway bridges. He has lately under- 

 taken a systematic investigation, which has resulted in the confirmation of his 

 theory. For experiment he took from the Wabash dam, at Delphi, a number of 

 bolts and spikes, which were, when the dam was constructed, of the best quality of 

 malleable bar iron, as is shown by the battering of the head when they were put 

 into the structure. Of these bolts and spikes he found that seventy per cent of 

 the whole number were as weak as cast iron, while ninety per cent of those which 

 were near the bottom of the dam were worthless; yet of those which were rotten, 

 the tips where inserted in immoveable rocks were fibrous and strong. When 

 broken they showed polished ends to the connecting fibers, indicating that the 

 continued vibrations of many years had polished and rounded the points of fibrous 



