THE REFRACTION OF SOUND. 213 



waves of soimd are frequently subjected. He remarks: "Although 

 barometric pressure does uot affect the velocity of sound, yet, as is well 

 kuowu, the velocity of sound depends on the temperature, and every 

 degree of temperature between 32° and 70° adds approximately one 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 appreciable result. This vapor nevertheless plays 

 an important part in the phenomena under consideration ; for it gives 

 to the air a much greater power of radiating and absorbing heat, and 

 thus renders it much more susceptible of changes in the action of the 

 sun. ... It is a well-known fact that the temperature of the air 

 diminishes as we proceed upward, and that it also contains less vapor. 

 Hence it follows that, as a rule, the waves of sound must travel faster 

 below than they do above, and thus be retracted or turned upward." — 

 {Proc. E. S., 1874.) 



Professor Eeyuolds cites observations showing that on a calm clear 

 day in July, 1873, while the sun was shining with great power, loud 

 sounds which could be heard but two or three miles were heard several 

 times this distance toward evening after the sun had become obscured 

 with clouds. " Here we see that the very conditions which actually 

 diminished the range of sound were precisely those which would cause 

 the greatest lifting of the waves." 



This furnishes a satisfactory explanation of the familiar fact that 

 sounds heard during the day-time to comparatively short distances (es- 

 pecially in summer and with still air) are audible many times as far in 

 the night. " Humboldt could hear the falls of Orinoco three times as 

 loud by night as by day at a distance of one league ; and he states that 

 the same phenomenon has been observed near every waterfall in 

 Europe." Humboldt also remarked that the heating effect of the sua 

 was so great that " all distant objects had wavy undulating outlines, 

 the optical effect of the mirage. Not a breath of air moved the dust- 

 like sand. The sun stood in the zenith." — ( Vicics of Nature, Bohn's ed., 

 p. 200.) Dr. Gregory, in his experiments on sound, undertaken in 182-4, 

 observed that, on January 9, in the evening, with no wind stirring, 

 " the sound of the same charge fired from the same musket was heard 

 much more intensely on this clear frosty night than in the day-time of 

 January 3, at the same distance, 3,G00 feet." — {Phil. Mag., 1824, Ixiii, 

 404.) 



Fig. 5 illustrates this effect of heated lower strata of air in tilting 

 up the beams of sound in all directions. If we suppose the horizontal 

 lines to mark spaces upward, of 100 yards each, into which the air is 

 arranged by strata of diminishing temperatures of 3 degrees each, but 

 increasing more rapidly near the surface, (75°, 70°, 07^, 04°,) then near 

 the ground (at 75°) the liorizontal sound-beams will travel 5 feet per 

 second (or 23.5 feet per mile) faster than at the line n of 70°, or the 

 height of 300 feet ; at this line, 3 feet per second (or 14 feet per mile) 



