464 WRITINGS OF JOSEPH HENRY. [1875 



on sound moving with the wind. Another experiment 

 relative to the same class of phenomena was made on the 

 19th of August (see Fig. 4), the wind being S. S. W. Two 

 observers — General Woodruff, and Dr. Welling, starting from 

 the bottom of the cliff immediately below the light-house, 

 went along the beach, the one in the direction Aj, and the 

 other in direction A e. General Woodruff found that the 

 sound of the siren was distinctly heard all the way to the 

 breakwater, and was so loud that it probably could have 

 been heard for several miles in that direction. Dr. Welling — 

 on the contrary, entirely lost the sound within a quarter of 

 a mile of the light-house. This result is readily explained as 

 a case of lateral refraction ; the wind was in the direction 

 traversed by General Woodruff, and contrary to that pur- 

 sued by Dr. Welling. In the one case the wind — retarded 

 by the surface of the cliff, moved with less velocity than it 

 did farther out, and consequently the sound was thrown 

 against the face of the cliff, and on the ear of the observer, 

 and in the other thrown from it, thus leaving as it were a 

 vacuum of sound. The effect in this case was very striking, 

 since the siren was pointed toward the zenith, and the sound 

 in still air could have been heard for miles in every direction. 



Investigations as to the Effect of Wind on Audibility. — These 

 observations were made by the aid of two steamers. Captain 

 Walker, naval secretary of the board, having completed a 

 series of inspections in the third district, sent the steamer 

 Putnam, under Captain Fields, to aid the Mistletoe in the in- 

 vestigations. They were commenced on the 9th of August, 

 at 12 o'clock. The wind was S. S. W. with a velocity of 7^ 

 miles per hour. Barometer, 303 inches ; thermometer, dry 

 bulb, 74° F.; wet bulb, 71-5° F. 



The two steamers started from a buoy near the north end 

 of the island, the one steaming against the wind, and the 

 other with it, each blowing its whistle every minute. The 

 distance travelled by each steamer was estimated by the 

 running time, which from previous observations was found 

 to be ten miles per hour. Each vessel was furnished with 

 a whistle of the same size, of 6 inches diameter, actuated by 



