1874] WRITINGS OF JOSEPH HENRY. 437 



to the sound at the surface of the earth, the velocity gradu- 

 ally diminishing as we ascend, for in this case also the incli- 

 nation of the sound waves would be downward. 



Observations September 23, 1874. — The third series of inves- 

 tigations was made in company with Captain .John L. Davis 

 and Major Peter C. Hains, both of the Light-House Board, 

 and General I. G. Woodruff, engineer of the third district, 

 and Mr. Brown, patentee of the siren. For the purpose, three 

 light-house tenders were employed, viz: the Mistletoe, Captain 

 Keeney; the Pa^/iam, Captain Field; and the Coccus, Captain 

 Latham. 



The place of operation chosen for the first day's series was 

 about 1^ miles from the northern point of Sandy Hook. 



From the experience gained by the accumulated observa- 

 tions which had been made, it was concluded that the phe- 

 nomena of sound in regard to perturbing influences could 

 not be properly studied without simultaneously observing 

 the transmission of sound in opposite directions. It was 

 therefore concluded to employ at least two steamers in mak- 

 ing the investigations. 



In regard to this point the commission was fortunate in 

 being able to command the use, for a limited period, of the 

 three tenders mentioned above, which happened to be at the 

 time assembled at the light-house depot, Staten Island, and 

 could be spared from their ordinary operations for a few days 

 without detriment to the service. It was also fortunate in 

 selecting for the scene of the investigations an unobstructed 

 position in the lower bay of New York, and perhaps still 

 more fortunate in the season of the year when on account of 

 the heat of the sun — a land and sea breeze which changed 

 their directions at a particular hour of the day, enabled 

 results to be obtained bearing especially on the phenomena 

 to be investigated. 



Attention was first given to the character of the several 

 steam-whistles which were intended to be used as the sources 

 of the sound during the series of investigations. 



These whistles, which were sounded during the whole of 

 the observations with twenty pounds of steam on each boiler, 



