346 BULLETIN OF THE 



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 New 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 run- 

 ning 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 



In October, 1861, a series of observations was made at Sandy 

 Hook (New Jersey) with various instruments. A sound reflector 

 being employed, the distance at which the sand on the phonometer 

 drum — carried in front, ceased to move was 51 yards, as compared 

 with a distance of 40 yards, without the reflector. At a greater 

 distance, with a more sensitive instrument, the ratio was very much 

 diminished, Experiments were also made on the relative distances 

 at which the trumpet affected sensibly the drum of the phonometer 

 in different directions, giving as their result a limiting spheroid 

 whose reach in the forward axis of the trumpet was about double 

 that in the rear axis, and at right angles to the axis, was about a 

 mean proportional between the two. With greater distances, 

 these differences were evidently very much reduced, the radii 

 becoming more equalized. In the summer of 1871, Henry made 



* Report Brit. Assoc, vol. xxiv. 2d part, p. 27. 



t Report of Li gilt House Board, U. S. for 1874, p. 92. 



X This difference lias since been established by a number of independ- 

 ent observations. Mr. Glaisher from his balloon ascents in 18(j3-1865, 

 ascertained that the upper currents of air are frequently five or six times 

 more rapid than the surface currents. {Travels in the Air, p. 9.) Prof. 

 Cleveland Abbe remarks : •' From seven balloon ascensions made on July 

 4th, 1871, at different points in the United States, I have deduced the 

 velocity of the upper currents as about four times that of the surface 

 wind prevailing." (^Bulletin Pkilosoph. Soc. Washington, Dec. 16, 1871, 

 vol. i. p. 39.) And M. Pesliu states in general terms : " It is certain 

 according to all observations made both in mountains and in balloons, 

 that the force of the wind increases considerably as we ascend in the 

 atmosphere." (Bulletin International de I'Observ. dt Paris et de V Observ. 

 Phys. Cent. Montsouris, July 7, 1872.) 



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