346 MEMORIAL OF JOSEPH HENRY. 



siderable distance. At the distance of a mile or two a large steam 

 whistle placed in the focus of a concave reflector 10 feet in diameter 

 could be heard very nearly as well directly behind the reflector, as 

 directly in front of it. In like manner the direction of bell- 

 mouths and of trumpet-mouths, was found to be of comparatively 

 little importance at a distance; showing the remarkable tendency 

 to diffusion, especially with very loud sounds. Most of the obser- 

 vations made on ship-board were afterward repeated on land; and 

 several weeks were occupied with these important researches. 



"During this series of investigations an interesting fact was dis- 

 covered, namely, a sound moving against the wind, inaudible to the 

 ear on the deck of the schooner, was heard by ascending to the 

 mast-head. This remarkable fact at first suggested the idea that 

 sound was more readily conveyed by the upper current of air than 

 the lower." After citing observations by others apparently con- 

 firming the suggestion of some dominant influence in the upper 

 wind, Henry adds : " The full significance however of this idea did 

 not reveal itself to me until in searching the bibliography of 

 sound, I found an account of the hypothesis of Professor Stokes in 

 the Proceedings of the British Association for 1857,* in which the 

 effect of an upper current in deflecting the wave of sound so as to 

 throw it down upon the ear of the auditor, or directing it upward 

 far above his head, is fully explained." f A rough attempt was 

 made in the course of these observations (which were undertaken 

 at the Light-house near ^ew Haven, Connecticut) to compare the 

 velocity of the wind in the upper regions with that near the surface 

 of the earth. " The only important result however was the fact 

 that the velocity of the shadow of a cloud passing over the ground 

 was much greater than that of the air at the surface, the velocity 

 of the latter being determined approximately by running a given 

 distance with such speed that a small flag was at rest along the side 

 of its pole. While this velocity was not perhaps greater than six 

 miles per hour, that of the shadow of the cloud was apparently 

 equal to that of a horse at full speed." J 



* Report Brit. Assoc. Dublin, 1857, vol. xxvii. 2d part, pp. 22, 23. 

 f Report of Light-House Board for 1874, p. 92. 



JThis difference has since been established by a number of independent 

 observations. Mr. Glaisher from his balloon ascents in 1863-1865, ascertained that 



