PHILOSOPHICAL SOCIETY OP WASHINGTON. 113 



opens into a vertical ventilating shaft through the rock, of up- 

 wards of a thousand feet in height. The top of this shaft opens 

 between two ridges of the Hoosac mountain, which rise respec- 

 tively some 400 and 'TOO feet higher. From the middle of the 

 tunnel when entirely clear of smoke, the distant opening at either 

 end appears as a faint star. The darkness seems oppi'essive ; 

 and when a train is passing through, the air becomes so thickly 

 clouded that the glare of torches cannot be seen at more than a 

 dozen feet distance. 



It had been constantly observed by those employed in the tun- 

 nel, that daring the approach of a locomotive at no great distance, 

 and a few minutes afterward, the sound of the engine was very 

 much deadened and obstructed ; so much so indeed, as to imperil 

 the workmen engaged in lining the top of the tunnel with a brick 

 arch, who frequently failed to hear the locomotive until it was 

 close upon them. This obscuration of sound was not uunatui'ally 

 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 appre- 

 ciable 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 striking experiments of the 

 distinguished physicist of the Royal Institution. A very simple 

 method of confirming this explanation, 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 con- 

 venient opportunity. The result has not yet transpired. 



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 tem- 

 perature is higher than the internal), it was observed that a pro- 

 longed but irregular echo followed any loud noise, such as the 



