1875] WRITINGS OF JOSEPH HENRY. 477 



ber balloons could be inflated at the time they were wanted. 

 Unfortunately this apparatus did not arrive in time to be of 

 much avail in this series of experiments. Besides this, on 

 account of the smallness of the balloons, the ascent was too 

 slow compared to the horizontal motion to indicate the 

 direction of the wind at a considerable elevation above the 

 points of observation. They were however of use in point- 

 ing out definitely the direction of the wind and the changes 

 it was undergoing. Moreover, at the time of leaving New 

 York we were able to procure only one anemometer, whereas 

 we ought to have had a number, one for the top of the 

 tower, one for the bottom, and one for each vessel. 



Experiments of September 3. — Barometer, 3002 inches ; 

 thermometer, dry bulb, 725° F. ; wet bulb, 70° ; wind from 

 the east, but too slight to move the cups of the anemometer ; 

 it soon however sprang up from the opposite direction, in 

 which it continued during the remainder of the day, attain- 

 ing a velocity of five and a quarter miles per hour. 



In these experiments two light-house steamers were em- 

 ployed, the Mistletoe and Cactus, which enabled us to obtain 

 the results in half the time, and thus to obviate in some 

 degree the effect of any change in the direction of the wind. 

 On this occasion the sound was noted at the light-house as 

 it converged to a centre from the whistle of each vessel, and 

 also simultaneously by each vessel as it diverged from the 

 vertical siren. 



We were enabled in this way to produce two curves by a 

 reverse process. These are plotted in Fig. 8, and exhibit a 

 remarkable degree of similarity. The corresponding parts 

 of the two curves, being in each case reversed, exhibit the 

 fact that through the same space in opposite directions the 

 audibility of the sound was similarly increased with the 

 wind and diminished against it. The effect however of the 

 wind in the experiments of this day was less marked than 

 on any in the whole series, and consequently the two curves 

 of audition more nearly approximate circles. 



We can see in this result no other effect than that which 

 would be produced from a wind flowing with a uniform but 



