« PreviousContinue »
the medium through which it has to pass, whatever tends to increase the elasticity of the air must also accelerate the motion of sound; on that account its velocity is greater in warm than in cold weather, supposing the pressure of the atmosphere constant. In dry air, at the freezing temperature, sound travels at the rate of 1089 feet in a second, and at 62° of Fahrenheit, its speed is 1123 feet in the same time, or 765 miles an hour, which is about threefourths of the diurnal velocity of the earth's equator. Since all the phenomena of sound are simple consequences of the physical properties of the air, they have been predicted and computed rigorously by the laws of mechanics. It was found, however, that the velocity of sound, determined by observation, exceeded what it ought to have been theoretically by 173 feet, or about one-sixth of the whole amount. La Place suggested that this discrepancy might arise from the increased elasticity of the air, in consequence of a development of latent heat during the undulations of sound, and the result of calculation fully confirmed the accuracy of his views. The aërial molecules being suddenly compressed give out their latent heat, and as air is too bad a conductor to carry it rapidly off, it occasions a momentary and local rise of temperature, which increasing the consecutive expansion of the air, causes a still greater development of heat, and as it exceeds that which is absorbed in the next rarefaction, the air becomes yet warmer, which favors the transmission of sound. Analysis gives the true velocity of sound, in terms of the elevation of temperature that a mass of air is capable of communicating to itself, by the disengagement of its own latent heat, when it is suddenly compressed in a given ratio. This change of teinperature, however,
cannot be obtained directly by experiment; but by invert ing the problem, and assuming the velocity of sound as given by experiment, it was computed that the temperature of a mass of air is raised nine-tenths of a degree when the compression is equal to of its volume.
Probably all liquids are elastic, though considerable force is required to compress them. Water suffers a condensation of nearly 0-0000496 for every atmosphere of pressure, and is consequently capable of conveying sound even more rapidly than air, the velocity being 4708 feet in a second. A person under water hears sounds made in air feebly, but those produced in water very distinctly. According to the experiments of M. Colladon, the sound of a bell was conveyed under water through the Lake of Geneva to the distance of about nine miles. He also perceived that the progress of sound through water is greatly. impeded by the interposition of any object, such as a projecting wall; consequently sound under water resembles light, in having a distinct shadow. It has much less in air, being transmitted all round buildings, or other obstacles, so as to be heard in every direction, though often with a considerable diminution of intensity, as when a carriage turns the corner of a street.
The velocity of sound, in passing through solids, is in proportion to their hardness, and is much greater than in air or water. A sound which takes some time in travelling through the air, passes almost instantaneously along a wire six hundred feet long, consequently it is heard twice, first as communicated by the wire, and afterwards through the medium of the air. The facility with which the vibrations of sound are transmitted along the grain of a log of wood is well known; indeed they pass through
iron, glass, and some kinds of wood at the rate of 18530 feet in a second. The velocity of sound is obstructed by a variety of circumstances, such as falling snow, fog, rain, or any other cause which disturbs the homogeneity of the medium through which it has to pass. Humboldt says, that it is on account of the greater homogeneity of the atmosphere during the night that sounds are then better heard than during the day, when its density is perpetually changing from partial variations of temperature. His attention was called to this subject by the rushing noise of the great cataracts of the Orinoco, which seemed to be three times as loud during the night as in the day, from the plain surrounding the Mission of the Apures. This he illustrated by the celebrated experiment of Chladni. A tall glass, half full of champagne, cannot be made to ring as long as the effervescence lasts; in order to produce a musical note, the glass, together with the liquid it contains, must vibrate in unison as a system, which it cannot do, in consequence of the fixed air rising through the wine and disturbing its homogeneity, because the vibrations of the gas being much slower than those of the liquid, the velocity of the sound is perpetually interrupted. For the same reason, the transmission of sound as well as light is impeded in passing through an atmosphere of variable density. Sir John Herschel, in his admirable Treatise on Sound, thus explains the phenomenon. It is obvious, he says, 'that sound as well as light must be obstructed, stifled, and dissipated from its original direction by the mixture of air of different temperatures, and consequently elasticities; and thus the same cause which produces that extreme transparency of the air at night, which astronomers alone fully appreciate, renders it also more
favorable to sound. There is no doubt, however, that the universal and dead silence, generally prevalent at night, renders our auditory nerves sensible to impressions which would otherwise escape notice. The analogy between sound and light is perfect in this as in so many other respects. In the general light of day the stars disappear. In the continual hum of voices, which is always going on by day, and which reach us from all quarters, and never leave the ear time to attain complete tranquillity, those feeble sounds which catch our attention at night make no impression. The ear, like the eye, requires long and perfect repose to attain its utmost sensibility.'
Many instances may be brought in proof of the strength and clearness with which sound passes over the surface of water or ice. Lieutenant Foster was able to carry on a conversation across Port Bowen harbor, when frozen, a distance of a mile and a half.
The intensity of sound depends upon the extent of the excursions of the fluid molecules, on the energy of the transient condensations and dilations, and on the greater or less number of particles which experience these effects; and we estimate that intensity by the impetus of these fluid molecules on our organs, which is consequently as the square of the velocity, and not by their inertia, which is as the simple velocity; for were the latter the case, there would be no sound, because the inertia of the receding waves of air would destroy the equal and opposite inertia of those advancing, whence it may be concluded, that the intensity of sound diminishes inversely as the square of the distance from its origin. In a tube, however, the force of sound does not decay as in open air, unless, perhaps, by friction against the sides. M. Biot found, from a num
ber of highly interesting experiments which he made on the pipes of the aqueducts in Paris, that a continued conversation could be carried on, in the lowest possible whisper, through a cylindrical tube about 3120 feet long, the time of transmission through that space being 2.79 seconds. In most cases sound diverges in all directions; but a very elegant experiment of Dr. Young's shows that there are exceptions. When a tuning-fork vibratés, its two branches alternately recede from and approach one another; each communicates its vibrations to the air, and a musical note is the consequence. If the fork be held
upright, about a foot from the ear, and turned round its axis while vibrating, at every quarter revolution the sound will scarcely be heard, while at the intermediate points it will be strong and clear. This phenomenon is occasioned by the air rushing between the two branches of the fork when they recede from one another, and being squeezed out when they approach, so that it is in one state of motion in the direction in which the fork vibrates, and in another at right angles to it.
It appears from theory as well as daily experience, that sound is capable of reflection from surfaces, according to the same laws as light. Indeed any one who has observed the reflection of the waves from a wall on the side of a river, or very wide canal, after the passage of a steamboat, will have a perfect idea of the reflection of sound and of light. As every substance in nature is more or less clastic, it may be agitated according to its own law, by the impulse of a mass of undulating air; but recipro cally, the surface by its reaction will communicate its undulations back again into the air. Such reflections produce echoes, and as a series of them may take place be