1877] WRITINGS OF JOSEPH HENRY. 501 



tached and returned along the same line, while the train was 

 drawn the remaining distance along the eastern decline by a 

 single engine. In order to make the experiment with regard 

 to sound, the time was accurately noted during which the noise 

 of the entering engines could be distinctly heard, which 

 would give approximately the distance the sound travelled 

 through the flocculent atmosphere produced by the locomo- 

 tive before becoming inaudible, and again the time was 

 noted from the first hearing of the returning engine until it 

 reached the end of the tunnel. In the meantime the cur- 

 rent of air blowing through the tunnel had removed a con- 

 siderable portion at least — of the flocculent atmosphere, so 

 that the sound in this case came through an atmosphere of 

 comparatively uniform temperature, or one much less floccu- 

 lent than the other ; the result was that the duration of sound 

 in the first case was about a minute, while in the second it 

 was upward of two minutes. The darkness in the tunnel — 

 on account of the smoke, was so profound — immediately after 

 the passage of a locomotive, that with two large torches, charged 

 with mineral oil, the sides of the tunnel at a distance of six 

 feet could scarcely be observed ; while in the other half of 

 the tunnel, where no smoke existed, the eastern opening could 

 be observed like a star at the distance of upward of two miles. 

 It was therefore not surprising that the stifling of the sound 

 which was observed should be referred to the smoke as a pal- 

 pable cause, and that the more efiicient one of the varying 

 density or flocculent condition should be disregarded. 



The method of determining by experiment the question 

 as to which of these causes was the efficient one did not 

 occur to me until we had left the tunnel, and then the sim- 

 ple expedient suggested itself to me for the purpose of repeat- 

 ing the experiment, that instead of locomotives charged with 

 wood, two locomotives charged with charcoal or coke — which 

 emits no smoke, but only transparent gases, principally car- 

 bonic acid, — should be used in an experiment similar to the 

 one just described. This experiment Mr. Locke has kindly 

 promised to perform as soon as it can conveniently be ar- 

 ranged. 



