480 



WRITINGS OF JOSEPH HENRY. 



[1875 



the Cactus. To explain the abnormal line 6 c, we have only 

 to suppose that a change in the velocity of the wind after- 

 ward took place by which its opposition to the sound-wave 

 was diminished; this will account for the greater length of 

 the line ; the change however did not reach the light-house 

 until after the vessel had passed the point b. 



As affording evidence in support of this hypothesis, it may 

 be mentioned that on examining the records of the Signal- 

 Service, (of which there is a station at New London, seven 

 miles north of the position at which these observations were 

 made,) it was found that the wind in the morning of that 

 day was south, in the afternoon south-west, and in the even- 

 ing north-west, and that it was probable as in other cases 

 that the wind had changed above while the part of the course 

 b c was being run. 



Experiments of September 6. — Barometer, 29'93 inches; 



^^-%. 



^^%. 



Fig. 10. 



thermometer, dry bulb, 745° F.; wet bulb, 67°; wind from 

 north-westto south-west, seventeen miles per hour. The wind, 



