Refraction of Sound by the Atmosphere. 541 



even more marked than before ; for at 90 yards to windward, with the 

 bell raised, I could hear it much more distinctly than at a corresponding 

 distance to leeward. This fact calls for a word of special explanation ; it is 

 clearly due to the fact that the variation in the velocity of the air is much 

 greater near the ground than at a few feet above it. When the bell is 

 on the ground all the sound must pass near the ground, and will all be 

 turned up to a nearly equal extent ; but when the bell is raised, the rays 

 of sound which proceed horizontally will be much less bent or turned up 

 than those which go down to the ground ; and consequently, after pro- 

 ceeding some distance, these rays will meet or cross, and if the head be 

 at this point they will both fall on the ear together, causing a sound of 

 double intensity. It is this crossing of the rays also which for the most 

 part causes the interference seen in fig. 2. 



These experiments establish three things with regard to the transmis- 

 sion of sound : 



1. That when there is no wind, sound proceeding over a rough surface 

 is more intense above than below. 



2. That as long as the velocity of the wind is greater above than 

 below, sound is lifted up to windward and is not destroyed. 



3. That under the same circumstances it is brought down to leeward, 

 and hence its range extended at the surface of the ground. 



These experiments also show that there is less variation in the velocity 

 of the wind over a smooth surface than over a rough one. 



It seems to me that these facts fully confirm the hypotheses propounded 

 by Prof. Stokes, that they place the action of wind beyond question, and 

 that they afford explanations of many of the anomalous cases that have been 

 observed ; for instance, that sounds can be heard much further over water 

 than over land, and also that a light wind at sea does not appear to affect 

 sound at all, the fact being that the smooth water does not destroy either 

 tire .sound or the motion of the air in contact with it. When the wind 

 and sea are rough the case is different. 



The Effect of Variations of Temperature. 



Having observed how the wind acts to lift the waves of sound by 

 diminishing their velocity above compared with what is below, it was 

 evident to me that any other atmospheric cause which would diminish 

 the velocity above or increase that below would produce the same effect, 

 viz. would cause the waves to rise. 



Such a cause must at certain times exist in the variation in the condi- 

 tion of the air as we proceed upwards from the surface. 



Although barometric pressure does not affect the velocity of sound, 

 yet, as is well known, the velocity of sound depends on the temperature*, 



* It varies as the square root of ^j^ ~, and consequently as the square root of the 

 absolute temperature. 



