THE REFRACTION OF SOUND. 215 



lifted as it usuall^^ is, was brought dowu, and thus intensified at the 

 surface of the water, which, being perfectly smooth, was thus converted 

 into a sort of whispering-gallery." — {Proc. E. S., 1876.) 



This action is illustrated by Fig. 6, in which the temperature of the 

 air below the horizontal line q r, being gradually less toward the earth 

 than above the line, the sound-waves originating at s are shortened lat- 



FiG. 6. — Refractiou by cold. 



erally (or in the direction of the lower rays) about one foot per second 

 for each degree of refrigeration, and the sound-beams (always perpen- 

 dicular to the waves) are thus gradually bent downward. 



The remarkable distances to which sounds have sometimes been 

 heard in Arctic regions receive here a satisfactory explanation. " Lieu- 

 tenant Foster, in the third polar expedition of Captain Parry, found 

 that he could hold a conversation with a man across the harbour of Port 

 Bowen, a distance of 6,696 feet, or about a mile and a quarter." — (Sir J. 

 Herschel, Sound, sect. 21.) The same author remarks of the polar 

 regions : '' In consequence of the intense cold of the icy surface, con- 

 trasted, as it sometimes is in summer, with the warmth of the air, the 

 phenomena of atmospheric refraction are exaggerated in these regions 

 in a most extraordinary manner; the forms of ice-bergs, rocks, etc., are 

 seen drawn up in vertical altitude, and spread out at their apparent 

 summits laterally, so as to present no resemblance to their real form." — 

 (Sir J. Herschel, Physical Oeography, sect. 98.) From which we learn 

 that the optical deportment of the air may very often be accepted as an 

 index of its acoustic condition. 



In the play and interaction of these two great and prevalent modes 

 of acoustic refraction — that resulting from co-existent differences of 

 wind in varying directions and that from co-existent differences of tem- 

 perature — whether re-enforcing or checking each other, or leaving a dif- 

 ferential resultant, we have abundant opportunities to exercise the 

 judgment and discrimination of the most diligent observers. Professor 

 Eeynolds noticed that on some clear nights in May and June, 1875, 

 when a heavy dew indicated considerable refrigeration at the surface, 

 *' the sound could invariably be heard as far against a light wind as 

 with it," showing that the upward refraction from wind was completely 



