142 LEADING AMERICAN MEN OF SCIENCE 



sound the law was found to be altogether different the farther 

 the observer went away from the source, the less the influence 

 of the reflector, and at the distance of two or three miles the latter 

 was without effect, the sound being about equally audible in 

 whatever direction the reflector might be turned. Another impor- 

 tant discovery, made the following year, was that when a sound was 

 moving against the wind it might be heard at an elevation when 

 it was inaudible near the surface of the water. 



The observations resulted in collecting an immense mass of 

 facts, including many curious abnormal phenomena. Henry was 

 always extremely cautious in formulating theories of the subject, 

 and had no ambition of associating his name with a generalization 

 which future researches might disprove. The result of his obser- 

 vations, however, was to show that there were none of these curi- 

 ous phenomena which might not be accounted for by a species 

 of refraction arising from varying atmospheric currents. The 

 possible effects of this cause had been pointed out by Professor 

 Stokes of England in 1857, and the views of the latter seem to 

 have been adopted by Henry. One of the generalizations is very 

 clearly explained on this theory: A current of air is more rapid at 

 a short height above the water than at its immediate surface. If a 

 sound-wave is moving with such a current of air its upper part will 

 be carried forward more rapidly than its lower part; its front will 

 thus be presented downward and it will tend to strike the water. 

 If moving in an opposite direction against the wind, the greater 

 velocity of the latter above the water will cause the upper part of 

 the sound-wave to be retarded. The wave will thus be thrown 

 upward, and the course of the sound will be a curved line convex to 

 the water. Thus an observer at the surface may be in a region of 

 comparative silence, when by ascending a few yards he will reach 

 the region of sound vibration. 



It was at the lighthouse station in the month of December, 1877, 

 that Professor Henry noticed the first sympton of the disorder 

 which terminated his life a few months later. After passing 

 a restless and uncomfortable night, he arose in the morning, 

 finding his hand partially paralyzed. A neighboring physician 



