500 WRITINGS OF JOSEPH HENRY. [1877 



(as stated by the engineers,) was that it was greatly stifled 

 immediately after the passage of the locomotive, by the smoke 

 with which the air was filled at the time. 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 locomotive, 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 practically interrupted by fog, snow, rain, 

 or hail,) I was induced to attribute the effects which had 

 been observed to another cause, and to regard the phenome- 

 non as due to an exaggerated flocculent condition of the air 

 in the tunnel, adopting in this instance the hypothesis ad- 

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

 ious experiments. The effect 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 progress before the observer at a distance. This con- 

 sists of a long stream composed of a series of globular masses 

 produced by the successive puffs of steam which are emitted 

 at equal intervals. Allowing the diameter of the driving- 

 wheels to be five feet, then since four puffs 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 temperature 

 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 middle of the tunnel, where it was de- 



