PHILOSOPHICAL SOCIETY OF WASHINGTON. 349 



tlie two others were directed- in opposite courses, one witli the 

 wind, or eastward, tlie otiier against it, or westward. In 15 

 minutes the whistle of the former ceased to be heard, while that 

 of the latter was very distinctly heard ; the anemometer showing 

 a wind of about six miles per hour. About noon the vessels 

 changed positions, but the sound from the west continued audible 

 lor about three times the distance of that from the east, though 

 the wind hud declined to nearly a calm or to about hiilf a mile 

 l)er hour. In an hour and a half the wind had changed to "within 

 two points of iiu exactly op[)osite direction, blowing from the 

 indications of the anemometer at the rate of ten and a half miles 

 l)er hour." The vessels once more dejjarting, one with the wind, 

 the other against it, the sound of the whistle coming against the 

 wind was this time heard for the greater distance, contrary to 

 expectation. On the following day a number of small balloons 

 having been provided, a similar series of experiments to that of 

 the preceding day was made ; a station being selected at a greater 

 distance from land. On the first trial, with a light wind from the 

 west of about one and a quarter miles per hour as indicated by the 

 anemometer, a balloon was set off which continued rising and 

 moving eastward till lost to sight. Two of the vessels taking 

 opposite courses as l)efore, gave the sound in the direction of the 

 wind about double the duration of that coming against the slight 

 wind. The vessels then changed places in their opposite courses ; 

 the wind having subsided to a calm. " A balloon let off ascended 

 vertically until it attained an elevation of about 1,000 feet, when 

 turning east it followed the direction of the previous one. In this 

 case the sound of the whistle coming from the east was heard 

 somewhat longer than the opposite one. At the third trial made 

 after noon, the wind had changed nearly one-third of the circle, its 

 force being about five miles per hour. The vessels once more 

 taking their courses with the wind and against it, ''several bal- 

 loons set off at this time were carried by the surface wind west- 

 wardly until nearly lost to sight, when they were observed to turn 

 oast, following the direction of the wind traced in the earlier 

 observations." In this case the sound was heard with the wind 

 very slightly farther than against it. It was thus shown that the 

 upper current of wind had remained constant throughout the day, 

 while the changing surface wind was apparently a land and sea 

 breeze "due to the heating of the land as the day advanced:" 

 and the varying behavior of the sound beams was easily explained 

 by the varying differences of velocity in their wave fronts at dif- 

 ferent heights. 



In 1875 Henry continued his observations at Block Island (R. 

 I.) and at Little Gull Island (Conn.). The southern light-house 

 on Block Island standing on the edge of a perpendicular cliff 152 

 feet above the sea level, and being itself 52 feet hi<>h (to its focal 

 plane), this poini was selected for making investigations on the 



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