AS A VEHICLE OF SOUND. 
195 
paused, and listened with all attention ; but neither horn nor whistle was heard. The 
guns were again signalled for ; five of them were fired in succession, but not one of them 
was heard. We steamed in on the same bearing to 2 miles, and had the guns fired 
point-blank at us. The howitzer and the mortar, with 3-lb. charges, yielded a feeble 
thud, while the 18-pounder was wholly unheard. Applying the law of inverse squares, 
it follows that, with air and sea, according to accepted notions, in a far worse condition, 
the sound at 2 miles distance on July 1 must have had more than forty times the intensity 
which it possessed at the same distance at 3 p.m. on the 3rd. 
“ Over smooth water,” says Sir John Herschel, “ sound is propagated with remark- 
able clearness and strength.” Here was the condition ; still with the Foreland so close 
to us, the sea so smooth, and the air so transparent, it was difficult to realize that the 
guns had been fired or the trumpets blown at all. Had the sound been converted by 
internal friction into heat? or had it been wasted in partial reflections at the limiting 
surfaces of non-homogeneous masses of air ? Sulphur in homogeneous crystals is exceed- 
ingly transparent to radiant heat, whereas the ordinary brimstone of commerce is highly 
impervious to it — the reason being that the brimstone of commerce does not possess 
the molecular continuity of the crystal, but is a mere aggregate of minute grains not 
in perfect optical contact with each other. Where this is the case, a portion of the heat 
is always reflected on entering and on quitting a grain : hence when the grains are 
minute and numerous this reflection is so often repeated that the heat is entirely wasted 
before it can plunge to any depth into the substance. The same remark applies to 
snow, foam, clouds, and common salt, indeed to all transparent substances in powder ; 
they are all impervious to 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 ; 
and he ascribed the deadening of the sound to the reflections which it endured at the 
limiting surfaces of the rarer and the denser air. This philosophical explanation, which 
admits of experimental illustration in the laboratory, made it generally known that a 
non-homogeneous atmosphere is unfavourable to the transmission of sound. 
But what on J uly 3, not with the variously heated plain of Antures, but with a calm 
sea as a basis for the atmosphere, could so destroy its homogeneity as to enable it to 
quench in so short a distance so vast a body of sound 1 ? I here submit to the judgment 
of scientific men my own course of thought regarding this question. As I stood upon 
the deck of the ‘ Irene ’ pondering it, 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 
2 c 2 
