626 WRITINGS OF JOSEPH HENRY. ["1877 



— is that in the spread or divergency of the sound, the di- 

 rection of the impulse turns through an angle of a little 

 more than 90°, so as to meet the surface even of the smooth 

 ocean in a direction by which it would be reflected to the 

 ear of the observer, making the angle of reflection equal to 

 the angle of incidence ; although from the gradual disper- 

 sion of sound-beams, the precise equality of these angles is 

 obviously not very important to the result. 



On returning from this excursion by the N. Y. Western 

 railway to the Hudson river at Troy, opportunity was 

 taken to make some observations on the action of sound in 

 the Hoosac tunnel, through which I passed, on the after- 

 noon of September 7th, accompanied by Mr. E. L. Woodruff. 

 Resting at East Windsor, near the western outlet, I spent a 

 considerable part of the following day in making an exami- 

 nation of the work. Mr. W. P. Granger, the chief engineer, 

 and Mr. A. W. Locke, his principal assistant, very courteously 

 furnished a hand-car, and cordially proffered every facility 

 for making any desired investigations. This tunnel (as is 

 known to most of those present) is nearly five miles long, 

 rising by an easy grade of 26*4 feet to the mile from either 

 mouth to about the middle of the tunnel, where it opens 

 into a vertical ventilating shaft through the rock — of upwards 

 of a thousand feet m height. The top of this shaft opens 

 between two ridges of the Hoosac Mountain, which rise 

 respectively some 400 and 700 feet higher. From the mid- 

 dle of the tunnel when entirely clear of smoke, the distant 

 opening at either end appears as a faint star. The darkness 

 seems oppressive; and when a train is passing through, the 

 air becomes so thickly clouded that the glare of torches can- 

 not be seen at a distance of more than a dozen feet. 



It had been constantly observed by those employed in the 

 tunnel, that during 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, v/ho frequently failed to 

 hear the locomotive until it was close upon them. This ob- 



