Refraction of Sound by the Atmosphere. 545 



were there, attesting the activity of the guns. Nothing was heard. We 

 went nearer ; but at two miles horns and whistles and guns were equally 

 inaudible. This, however, being near the limit of the sound-shadow, I 

 thought that might have something to do with the effect, so we steamed 

 right in front of the station, and halted at 3| miles from it. Not a 

 ripple nor a breath of air disturbed the stillness on board, but we heard 

 nothing. There were the steam-puffs from the whistles, and we knew 

 that between every two puffs the horn-sounds were embraced, but we 

 heard nothing. We signalled for the guns ; there were the smoke-puffs 

 apparently close at hand, but not the slightest sound. It was mere 

 dumb-show on the Foreland. We steamed in to 3 miles, halted, and 

 listened with all attention. Neither the horns nor the whistles sent us 

 the slightest hint of a sound. The guns were again signalled for ; five 

 of them were fired, some elevated, some fired point-blank at us. Not one 

 of them was heard. We steamed in to two miles, and had the guns 

 again fired : the howitzer and mortar with 3-lb. charges yielded the 

 faintest thud, and the 18-pounder 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 spe- 

 cial attention to this subject, that a clear, calm atmosphere was the best 

 vehicle of sound : optical clearness and acoustic clearness were supposed 

 to go hand in hand * * *. 



"As I stood upon the deck of the 'Irene' pondering this question, 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 considered in the highest degree 

 improbable. It would be sure, I thought, to streak and mottle the 

 atmosphere with spaces, in \vhich the air would be in different degrees 

 saturated, or it might be displaced by the vapour. At the limiting sur- 

 faces 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 ex- 

 planation 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 Fore- 

 land. 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 inaudibility. 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 previously 



