the Refraction of Sound by the Atmosphere. 75 



howitzer and mortar with 3-lb. charges yielded the faintest thud, 

 and the 18-pomider was quite unheard. 



" In the presence of these facts I stood amazed and confounded ; 

 for it had been assumed and affirmed by distinguished men who had 

 given special attention to this subject, that a clear, calm atmosphere 

 was the best vehicle of sound: optical clearness and acoustic clear- 

 ness were supposed to go hand in hand * * *. 



"As I stood upon the deck of the 'Irene' pondering this ques- 

 tion, I became conscious of the exceeding power of the sun beating 

 against my back and heating the objects near me. Beams of 

 equal power were falling on the sea, and must have produced copious 

 evaporation. That the vapour generated should so rise and mingle 

 with the air as to form an absolutely homogeneous mixture I con- 

 sidered in the highest degree improbable. It would be sure, I 

 thought, to streak and mottle the atmosphere with spaces, in 

 which the air would be in different degrees saturated, or it might 

 be displaced by the vapour. At the limiting surfaces of these 

 spaces, though invisible, we should have the conditions necessary 

 to the production of partial echoes, and the consequent waste of 

 sound. 



" Curiously enough, the conditions necessary for the testing of 

 this explanation immediately set in. At 3.15 p.m. a cloud threw 

 itself athwart the sun, and shaded the entire space between us 

 and the South Foreland. The production of vapour was checked 

 by the interposition of this screen, that already in the air being at 

 the same time allowed to mix with it more perfectly ; hence the 

 probability of improved transmission. To test this inference the 

 steamer was turned and urged back to our last position of inau- 

 dibility. The sounds, as I expected, were distinctly though faintly 

 heard. This was at 3 miles distance. At 3| miles we had the 

 guns fired, both point-blank and elevated. The faintest thud 

 was all that we heard ; but we did hear a thud, whereas we had pre- 

 viously heard nothing, either here or three quarters of a mile 

 nearer. We steamed out to 4| miles, when the sounds were for 

 a moment faintly heard, but they fell away as we waited ; and 

 though the greatest quietness reigned on board, and though the 

 sea was without a ripple, we could hear nothing. We could plainly 

 see the steam-puffs which announced the beginning and the end 

 of a series of trumpet-blasts, but the blasts themselves were quite 

 inaudible. 



" It was now 4 p.m., and my intention at first was to halt at this 

 distance, which was beyond the sound-range, but not far beyond it, 

 and see whether the lowering of the sun would not restore the 

 power of the atmosphere to transmit the sound. But after waiting 

 a little, the anchoring of a boat was suggested ; and though loth to 

 lose the anticipated revival of the sounds myself, I agreed to this 

 arrangement. Two men were placed in the boat, and requested to 

 give all attention, so as to hear the sound if possible. With perfect 

 stillness around them, they heard nothing. They were then in- 

 structed to hoist a signal if they should hear the sounds, and to 

 keep it hoisted as long as the sounds continued. 



" At 4.45 we quitted them and steamed towards the South Sand 



