ADDRESS OF PROF. S. NEWCOMB. 465 



so that at snort distances it is many times louder in front of the 

 reflector than behind it. In the case of light, which moves in right 

 lines, it is well known that such an increased volume of light thrown 

 in one direction will go on indefinitely. But in the case of 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 important dis- 

 covery, 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. 



These observations were continued from time to time during the 

 summer season until 1877. They resulted in collecting an immense 

 mass of facts, including many curious abnormal phenomena, descrip- 

 tions of which are found in the annual Reports of the Light-House 

 Board. Our president was 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 observations however was to show that there were none of 

 these curious phenomena which might not be accounted for by 

 a species of refraction arising from varying atmospheric cur- 

 rents. 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 generaliza- 

 tions 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 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. A corresponding effect 

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