524 EESEARCHES IN SOUND. 



the cliff, moved with less velocity than it did farther out, and conse- 

 quently 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 the 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 at Block Island 

 in eegakd to the effect of wind on audibility. 



These 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 investigations. 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, 30.3 inches; thermometer, dry bulb, 74P F. 

 wet bulb, 71io 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 esti- 

 mated 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 the same pressure of 20 

 pounds of steam, and which, by previous comparison, had been found to 

 give sound at this pressure of the same penetrating power. The obser- 

 vations on the Mistletoe were made by General Woodruff", and on the 

 Putnam by Dr. Welling, each assisted by the officers of the respective 

 vessels. The two steamers i^roceeded to buoy off" the north end of the 

 island, in which position the wind was unobstructed by the land — a low 

 beach. Indeed, the island being entirely destitute of trees, and consist- 

 ing of a rolling surface, the wind had full sweep over it in every direction. 



First experiment. — The Putnam went against the wind and the Mistle- 

 toe in the opposite direction. The Putnam lost the sound of the whistle 

 of the Mistletoe in two minutes and stopped, but continued to blow the 

 whistle. The Mistletoe continued on her course and heard the Putnam's 

 whistle for twenty minutes in all. During the first two minutes both 

 vessels were in motion, and therefore the space through which the sound 

 was heard moving against the wind would be represented by 4, while the 

 space through which the sound was heard moving with the wind would 

 be represented by 20--1- 2 = (22), the ratio being 1 : 5J. 



Second experiment. — In this the Putnam went with the wind and the 

 Mistletoe in the opposite direction. The Mistletoe lost the sound of the 

 Putnam's whistle in two minutes. The Putnam then stopped and re- 

 mained at rest, while the Mistletoe continued on her course until the 

 Putnam lost sound of her whistle, twenty- six minutes later. As both 

 steamers were separating during the first two minutes with equal speed, 



