COMPARATIVE INTENSITY OF SOUNDS.

The inhabitants of Atures and Maypures, whatever the missionaries may have asserted in their works, are not more struck with deafness by the noise of the great cataracts, than the Catadupes of the Nile.

When this noise is heard in the plain that surrounds the mission, at the distance of more than a league, you seem to be near a coast skirted by reefs and breakers.

The noise is three times as loud by night as by day, and gives an inexpressible charm to these solitary scenes.

What can be the cause of this increased intensity of sound in a desert, where nothing seems to interrupt the silence of nature?

The velocity of the propagation of sound, far from augmenting, decreases with the lowering of the temperature.

The intensity diminishes in air, agitated by a wind, which is contrary to the direction of the sound; it diminishes also by dilatation of the air, and is weaker in the higher than in the lower regions of the atmosphere, where the number of particles of air in motion is greater in the same radius.

The intensity is the same in dry air, and in air mingled with vapours; but it is feebler in carbonic acid gas, than in mixtures of azot and oxygen.

From these facts, which are all we know with any certainty, it is difficult to explain a phenomenon observed near every cascade in Europe, and which, long before our arrival in the village of Atures, had struck the missionary and the Indians.

The nocturnal temperature of the atmosphere is 3° less than the temperature of the day; at the same time the apparent humidity augments at night, and the mist that covers the cataracts becomes thicker.

We have just seen, that the hygroscopic state of the air has no influence on the propagation of the sound, and that the cooling of the air diminishes its swiftness.

It may be thought, that, even in places not inhabited by man, the hum of insects, the song of birds, the rustling of leaves agitated by the feeblest winds, occasion, during the day, a confused noise, which we perceive the less because it is uniform, and constantly strikes the ear.

Now this noise, however slightly perceptible it may be, may diminish the intensity of a louder noise; and this diminution may cease, if during the calm of the night the song of birds, the hum of insects, and the action of the wind upon the leaves, be interrupted.

But this reasoning, even admitting its justness, can scarcely be applied to the forests of the Oroonoko, where the air is constantly filled by an innumerable quantity of moschettoes, where the hum of insects is much louder by night than by day, and where the breeze, if ever it be felt, blows only after sunset.

I rather think, that the presence of the sun acts upon the propagation and intensity of the sound by the obstacles which they find in the currents of air of different density, and the partial undulations of the atmosphere caused by the unequal heating of different parts of the soil.

In calm air, whether it be dry, or mingled with vesicular vapours equally distributed, the sonorous undulation is propagated without difficulty.

But when the air is crossed in every direction by small currents of hotter air, the sonorous undulation is divided into two undulations; where the density of the medium changes abruptly, partial echoes are formed, that weaken the sound, because one of the streams comes back upon itself; and those divisions of undulations take place, of which Mr.

Poisson has recently developed the theory with great sagacity.

It is not therefore the movement of the particles of air from below to above in the ascending current, or the small oblique currents, that we consider as opposing by a shock the propagation of the sonorous undulations.

A shock, given to the surface of a liquid, will form circles around the center of percussion, even when the liquid is agitated.

Several kinds of undulations may cross each other in water, as in air, without being disturbed in their propagation; little movements may ride over each other, and the real cause of the less intensity of sound during the day appears to be the interruption of homogeneity in the elastic medium.

During the day, there is a sudden interruption of density wherever small streamlets of air of a high temperature rise over parts of the soil unequally heated.

The sonorous undulations are divided, as the rays of light are refracted, and form the mirage (looming,) wherever strata of air of unequal density are contiguous.

The propagation of sound is altered, when a stratum of hydrogen gas is made to rise in a tube closed at one end above a stratum of atmospheric air; and Mr.

Biot has well explained, by the interposition of bubbles of carbonic acid gas, why a glass filled with Champagne wine is little sonorous so long as the gas is evolved, and continues to pass through the strata of the liquid.

Humboldt's Personal Narrative.