1877] WRITINGS OF JOSEPH HENRY. 527 



scuration of sound was not unnaturally attributed to the 

 dense clouds of smoke constantly emitted by the locomotive ; 

 but this explanation can hardly be accepted as the true one, 

 nor the condition noted as constituting even an appreciable 

 cause of such acoustic opacity. When we reflect that a puff 

 of exhaust-steam at high temperature is ejected at about 

 every four feet of rail traversed by the driving wheels, it is 

 not difficult to realize that in an atmosphere so systematically 

 made heterogeneous there must be a very great amount of 

 dispersion and absorption of sound waves struggling through 

 such a medium. This has been well illustrated by the strik- 

 ing experiments of the distinguished physicist of the Royal 

 Institution. A very simple method of confirming this ex- 

 planation, and of eliminating entirely the effect of the smoke 

 would be the employment of locomotive engines driven by 

 the combustion of coke or of charcoal. This experimental 

 determination of the question did not occur to me till after 

 we had left the tunnel; but on suggesting it to Mr. A. W, 

 Locke, the assistant engineer in charge, he very obligingly 

 undertook the conduct of such an experiment at the earliest 

 convenient opportunity. The result has not yet been ascer- 

 tained. 



When the tunnel was entirely clear, and a gentle current 

 of air flowing down the central ventilating shaft and out at 

 the two ends (as is usual in the summer season, when the 

 external temperature is higher than the internal), it was 

 observed that a prolonged but irregular echo followed any 

 loud noise, such as the sudden shutting down of the lid of a 

 tool-chest. The unequal or somewhat intermittent character 

 of the echo appeared to result from the irregular surface of 

 the rock forming the walls of the tube. A somewhat similar 

 echo is sometimes returned from the dense foliage of trees. 

 It is proper to add that a very perceptible echo was heard 

 from the portion of the tunnel lined with brick. The effect 

 could in neither case be ascribed to any invisible " floecu- 

 lence," as the air must have been in a very homogeneous 

 condition. 



Inasmuch as in such observations the waves of sound are 



