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ANNUAL REPORT SMITHSONIAN INSTITUTION, 19 35 



This last explanation seems quite satisfactory and its importance 

 is that it gives a method of measuring the temperature of the air at 

 the great heights reached by the sound waves. By using special 

 microphones instead of the ear it is possible to detect the waves from 

 the firing of one big gun at a distance of a hundred miles or more. 

 It is easy to measure the total time taken by the sound to travel 

 along its path to the upper atmosphere and back, and further, by a 

 suitable arrangement of three microphones at the receiving station, 

 it is possible to measure the angle at which the downcoming sound 

 ray strikes the earth's surface. Given such observations, it is not 



FiGUEB 1. — The passage of sound waves to great distances. (After Whipple.) The 

 diagram represents a vertical section of the atmosphere and shows two paths by 

 which sound waves may travel to great distances via the upper atmosphere. The 

 source of the sound is at the bottom left-hand corner. The sound waves are bent 

 upward in the lowest region and downward in the upper region owing to the opposite 

 temperature gradients in these regions. In the middle region of constant temperature 

 the sound travels straight. 



difficult to calculate most of the details about the path traveled by 

 the sound, such as the maximum height above the ground and the 

 speed of sound at that height. Then, since the speed depends on 

 the temperature of the air, it is possible to estimate the temperature 

 of the atmosphere at these great heights. 



Not infrequently two or more sets of downcoming waves are re- 

 corded at a great distance which all come from one single explosion, 

 the waves being separated from each other by several seconds and 

 the angle at which they strike the earth being dilfferent in each case. 

 In such cases it is clear that the waves have traveled through the 

 upper atmosphere by different paths, reaching different maximum 

 heights before finally converging on to the recording microphones. 



