550 THE POPULAR SCIENCE MONTHLY. 



light, not through the immediate absorption or extinction of the light, 

 but through repeated internal reflection. 



Humboldt, in his observations at the Falls of the Orinoco, is 

 known to have applied these principles to sound. He found the noise 

 of the falls far louder by night than by day, though in that region 

 the night is far noisier than the day. The plain between him and the 

 falls consisted of spaces of grass and rock intermingled. In the heat 

 of the day he found the temperature of the rock to be considerably 

 higher than that of the grass. Over every heated rock, he concluded, 

 rose a column of air rarefied by the heat ; its place being supplied by 

 the descent of heavier air. He ascribed the deadening of the sound 

 to the reflections which it endured at the limiting surfaces of the 

 rarer and denser air. This philosophical explanation made it gen- 

 erally known that a non-homogeneous atmosphere is unfavorable to 

 the transmission of sound. 



But what, on July 3d, not with the variously-heated plain of An- 

 tures, but with a calm sea as a basis for the atmosphere, could so de- 

 stroy its homogeneity as to enable it to quench in so short a distance 

 so vast a body of sound ? My course of thought at the time was thus 

 determined. As I stood upon the deck of the Irene, pondering the 

 question, I became conscious of the exceeding power of the sun beat- 

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

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

 oration. That the vapor generated should so rise and mingle with the 

 air as to form an absolutely homogeneous medium was in the highest 

 degree improbable. It would be sure, I thought, to rise in invisible 

 streams, breaking through the superincumbent air, now at one point, 

 now at another, thus rendering the air flocculent with wreaths and 

 striae, charged in different degrees with the buoyant vapor. At the 

 limiting surfaces of these spaces, though invisible, we should have the 

 conditions necessary to the production of partial echoes and the con- 

 sequent waste of sound. Ascending and descending air-currents, of 

 different temperatures, as far as they existed, would also contribute to 

 the effect. 



Curiously enough, the conditions necessary for the testing of this 

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

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

 the South Foreland. The heating of the water, and the production 

 of vapor, were suddenly checked by the interposition of this screen ; 

 hence the probability of suddenly-improved transmission. To test 

 this inference, the steamer was immediately 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 

 3f miles the guns were fired, both point-blank and elevated. The 

 faintest pop was all that we heard ; but we did hear a pop, whereas 

 we had previously heard nothing, either here or three-quarters of a 



