32 JOURNAL OF THE 



Those who are familiar with the map of South America, 

 where the older rocks have been decomposed for great 

 depths /;/ situ^ and where the }Ounger rocks are but imper- 

 fectly lithified over great areas, must recall the fact that 

 nearly all the streams have their longer tributaries on the 

 right, showing a left-hand deflection of the main streams. 



The cases cited serve my purpose of showing that wher- 

 ever the conditions permit the influence of the earth's rota- 

 tion is perceptible. 



THE STONE ARCH. 



BY WILLIAM CAIN, C. E. 



The theory of the voussoir arch has long exercised the 

 ingenuity of mathematicians, and it may prove interesting, 

 before giving the results of some recent investigations by 

 the writer, to give brief statements of some of the leading 

 theories that have been proposed, from time to time, as 

 indicating the path followed in such original investigations. 



As we should naturally expect, the theories proceed from 

 the simplest, where the arch is assimilated in its action to 

 a wedge, to the most complex, where the deformation of 

 each individual stone under stress is considered. 



In most of the theories hitherto proposed the arch 

 is regarded as inelastic and the stones infinitely strong, so 

 that the resultant thrust of one part of the arch against 

 another can take place along the very edge of a joint with- 

 out crushing ensuing. 



These simple hypotheses unfortunately do not express 

 the actual conditions, which involve the consideration of 

 the elasticity of all the materials entering into the con- 

 struction of the arch, the fit of the stones, thickness and 



