Изображения страниц

clear, sonorous musical note given out when the vessel was full of air.

This experiment has been introduced to explain the reason why sounds are more distinctly heard during the night than by day. That the clearness and intensity of sounds are increased during the night is known to every one. It may be partly attributed to the repose of the animal creation, but this is not sufficient to account for the fact. In a retired country resort the only sounds that strike the ear in the busiest hours of day, are the lowing of oxen, the bleating of sheep, the merry songs of birds, the indifferent whistle of the ploughman, and sometimes the pleasing peel of bells. But with what a freshness does the last burst upon the ear when heard at evening when the sun has sunk to rest! Humboldt says that he was particularly struck with the greatly increased intensity of sound during the night, when he heard the noise of the great cataracts of the Oroonoko in the plain which surrounds the mission of Apures. It is, he says, three times greater at night than at day, although the noises arising from animals are more numerous and louder at that time. There must then be a reason for the greater intensity of sound at night, altogether independent of the influence of animal life. During the day the temperature of the atmosphere is not uniform. The stratum in immediate contact with the earth is heated both by radiation and conduction, and consequently has a higher temperature than the strata above it. The effect of heat upon all gases is known to be expansion, and therefore they must become bulk for bulk lighter. As soon then as the air in contact with the earth is heated it rises, and that which is above descends into its place, and this also suffers the same



change. Currents of different densities are constantly ascending and descending, so that during the day the sound is not so well conducted as at night, when the temperature of the atmosphere becomes more uniform. In dense fogs, and in snow-storms, sounds are badly conducted, for the same reason. The effect of carpets, woollens of all kinds, and furs, in deadening sound, arises from the same cause. The openness of texture allows the intermixture of air with the fibre, and this is quite sufficient to account for the singular effect. The alteration of conducting power produced by the admixture of gases of different kinds has not yet been determined; but from what has been already stated, we may perceive that in the constitution of the atmosphere, the power of conducting sound must have been ordained by the Creator.


Many liquids have the property of conducting sound as well as gases and vapours. Water, which is always considered as the type of the class to which it belongs, evidently has a conducting power. Divers inform us that they are sensible of sounds when at great depths beneath the surface of water, and not only of those sounds which are created in that medium, but also of those produced in air. In the latter case they are less distinct, as might be supposed. Dr. Franklin made a familiar experiment, which may be easily repeated, to determine whether sound can be conducted by water. An assistant was stationed about half a mile from the Doctor, and made to strike two stones together under the surface; the sound was distinctly heard by the latter, when the head was

plunged into the same medium. Anderon's experiments on this subject are the most interesting with which we are acquainted. On one occasion he caused three persons to dive, and remain at a depth of about two feet below the surface; during which time he spoke to them as loud as he was able, and they heard him, but thought that he spoke very low. At another time he engaged a diver to descend with a bell in his hand, and the sound was distinctly heard in the water. These facts are sufficient to prove the conducting power of one liquid, and all others probably possess the same property.


Elastic solids are found to be better conductors of sound than liquids. It is well known the ticking of a watch may be distinctly heard at the end of a long piece of timber, opposite to that at which the watch is placed. If, however, instead of the watch, an assistant tap with the head of a pin, and so gently as not to be even heard by himself, it will be quite audible to the person who applies his ear at the opposite end. A still more interesting experiment, mentioned by Chladni, may be made in proof of the ready conducting power of elastic solids. Suspend a piece of metallic wire, about 600 feet long, in an horizontal position, and attach to one end a metal plate which when struck will give out a clear tone. If the opposite end of the wire be taken between the teeth, and an assistant then strike the plate, the sound will be immediately conducted by the wire, and more slowly by the air; the sound as conducted by the metal is immediately heard, while that transmitted by the air follows.




Although the conductors of sound are extremely various in their physical constitution, being either solids, liquids, or gases, they must possess some common properties to which the power of transmitting sound may be attributed. When a tense string is touched, or a bell is rung, a tremulous motion is produced through the mass of the substance. This is evidently occasioned by a limited displacement of particles. Let us take as simple a case as we can. Imagine a string to be a single row of indivisible particles, or, as they are sometimes called, molecules. The union between them is produced and maintained by that attractive power denominated the force of cohesion. Within a certain limit, the close connexion between these particles may be disturbed without being destroyed. Let us for example examine this fact in reference to any two of the chain of molecules. The cohesive force binds them together, but some external force stronger than the cohesive may turn them from the line of direction, and yet not so far as to prevent the cohesive force from bringing them back to their original position, when the external agency is removed. Two magnets suspended near to each other will soon arrange themselves by magnetic attraction-the north pole of the one being directed to the south pole of the other. Remove either of these poles from the right line in which it is placed, and the other will be also disturbed; but as soon as the external force is taken away, the magnetic attraction acts without control, and after a few vibrations backwards and forwards, the poles come to precisely the same position they occupied before dis


turbance. It is just so with the particles of a sounding body. The particles of all substances are not equally capable of disturbance, and for this reason they are not all equally good sounding bodies. The power of receiving an alteration of form, and of afterwards returning to the original condition, is called elasticity, and this property is essential to the production of sound. We may, however, be told that all elastic substances are not sonorous, and to meet this objection fully would occupy more space than ought to be given to such a subject in an elementary work. One or two remarks, however, will not be out of place.

The elasticity possessed by bodies may be of different kinds; thus, for instance, both dough and indian-rubber have the property, but the effect is very different in the two cases. We may suppose the particles of dough to be elastic, but the sphere of their elasticity is small; and when once drawn beyond it, they suffer permanent displacement. In indianrubber, on the other hand, the sphere is very large, so that we may draw it out to a great distance without bringing the particles out of the influence of mutual attraction. So again, the ease with which the particles are displaced may vary; a force which would have a great effect upon indianrubber would have little or none on a piece of metal. These and similar differences should be estimated when considering the reason why one substance is sonorous and another is not.

We must, however, now proceed, upon the fact that a sounding body is in a state of vibration, to illustrate the effect produced upon a fluid-conducting medium, or in other words, the condition of such a medium during the trans

« ПредыдущаяПродолжить »