552 RESEARCHES IN SOUND. 



half and direct the streain entirely out of the other entrance. At the 

 time of our visit, there was a gentle current flowing out of both ends. 



The oidy peculiarity of sound which had been observed, as stated by 

 the engineers, was that it was greatly stilled at the time by the smoke 

 with which the air was tilled immediately after the j)assage of a loco- 

 motive. So great was this in some cases that accidents were imminent 

 to the workmen, who are constantly occupied in the tunnel in lining the 

 crown of the arch with brick, by the sudden appearance of a locomo- 

 tive, the approach of which had not been heard. 



That the audibility of sound should be diminished by smoke was so 

 contrary' to previous conceptions on the subject, since sound is not prac- 

 tically interrupted by fog, snow, rain, or Jiail, that I was induced to 

 attribute the efi'ects which had been observed to another cause, and to 

 regard the phenomenon as due to an exaggerated flocculent condition 

 of the air in the tunnel ; adopting in this instance the hyi^othesis ad- 

 vanced by Dr. Tyndall, and so well illustrated by his ingenious ex- 

 periments. The eliect which would be produced in the condition of 

 the air in the tunnel by the passage of a locomotive is indicated by 

 the appearance of the emitted steam extending behind the smoke- 

 stack of a locomotive in rapid j)rogress before the observer at a dis- 

 tance. This consists of a long stream composed of a series of globu- 

 lar masses produced by the successive puffs of steam which are emitted 

 at equal intervals. Allowing the diameter of the di'iving-wheels to be 

 five feet, then since four pufis are made at each revolution of the wheels, 

 a puff of hot steam would be given out at every four feet travelled by 

 the engine, and these puffs mingling with the air at the ordinary tem- 

 ])erature would produce an exaggerated flocculent condition. On our 

 expressing a desire to witness the effect upon sound of the passage of a 

 locomotive through the tunnel, Mr. A. W. Locke, one of the engineers 

 who had charge of the western section, politely offered us the means of 

 experimenting on this point, and also of passing leisurely through the 

 tunnel on a hand-car. 



To observe the effect of a locomotive on the sound we took advantage 

 of the entrance of a freight train, impelled by two engines, the extra 

 one being necessary to drive the load up the inclined plane to the mid- 

 dle of the tunnel, where it was detached 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 re- 

 gard to sound tlie time was accurately noted during which the noise of 

 the entering engines could be distinctly heard, which would give aj)- 

 ])roximately the distance the sound travelled through the flocculent at- 

 mosphere produced by the locomotive before becoming inaudible, and 

 again the time was noted from th^ first hearing of the returning engine 

 until it reached the end of the tunnel. In the mean time the current of 

 air blowing through the tunnel had removed a considerable portion, at 

 least, of the flocculent atmosphere, so that the seund in this case came 



