the Refraction of Sound by the Atmosphere. 71 



and the intensifying of the sounds to windward by the raising of 

 the bell was 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 w T hen 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 trans- 

 mission 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 lee- 

 ward, 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 pro- 

 pounded 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 the 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 pro- 

 duce the same effect, viz. would cause the waves to rise. 



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

 condition 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*, and every degree of temperature between 32° and 70° 

 adds approximately 1 foot per second to the velocity of sound. 

 This velocity also increases with the quantity of moisture in the 

 air ; but the quantity is at all times too small to produce an appre- 

 ciable result. This vapour nevertheless plays an important part in 



* It varies as the square root of - ,?\ and consequently as the square root 

 of the absolute temperature. 



