circles; the former are generated around that point in concentric spheres; all deviations from these forms being in both cases due to mere special disturbing influences. This conception having been fixed in the mind, it is readily seen that sound-waves consist of alternate swellings (involving a rarefaction) and contractions (involving a condensation) of the air, propagated from the point of origin, and that the thickness (length) of each wave is measured by the distance between the curved surfaces corresponding to the periods of maximum swelling (rarefaction) and contraction (condensation). Within the wave-limits, the progress of sound-motion is by no means uniform; and, could we accurately trace the steps in variation, we could readily delineate the march of sound within the wave.
This result may be obtained approximately by attaching a small piece of copper-foil to one of the prongs of a tuning-fork, and quickly drawing this (while vibrating) across a plate of smoked glass. A very beautiful representation of this march of sound may also be obtained by operating with an organ-pipe, having a hole at the middle (nodal-point), which is covered by a thin elastic membrane, offering no impediment to the transmission of the undulations. Directly over this membrane a little box or capsule is placed, through which a current of illuminating gas is conducted to a jet burning in front of a revolving mirror. The sound-waves being communicated to the gas, give rise to a series of flame-pulsations. When the mirror revolves, the quiet flame is reflected as a continuous, the pulsating flame as a serrated, band of light.
If, at this point of the experiment, the aid of one of Helmholz's resonance-spheres be called in—the resonance-waves being conducted by a pipe through a box and membrane (like those already described), to a second gas-jet placed exactly under the first—the image in the mirror will be duplicated. The resonance-spheres (resonators) here mentioned are thin, hollow, brass globes, with two openings opposite to each other; one being furnished with a neck for attaching a pipe, the other serving as a mouth for receiving sound-impulses. They act by sympathy, as it were, taking up and resounding with a special note, and that only, the special character of the note depending upon the relative capacity of the sphere, and the size of the mouth.
As the waves of sound, propagated through a uniform medium, travel with uniform velocity, it follows that, when the pulsations transmitted to the first jet from the organ-pipe, and the pulsations transmitted to the second jet from the resonance-sphere, pass through equal lengths of air, they will be reflected from the revolving-mirror as coincident serrations. When, however, the pulsations from the organ-pipe are transmitted through a depth of air equal to one wave-thickness or length, and the pulsations from the resonance-sphere are transmitted through a depth, either less or more (and not an exact multiple) than the wave-thickness or length, the two serrated bands of light, reflected from the revolving-mirror, will not be coincident. If, starting with equal distances of the organ-pipe and resonance-sphere from the jets, that of the latter be gradually increased, the serrations of the two images will be at first coincident, then non-coincident; then, when a distance of two wave-thicknesses is reached, again coincident, then again non-coincident; each coincident corresponding to a distance equal to a simple multiple of the wave-length of the note. And if, on the other hand, the resonance-sphere be moved in such a manner that the coincidence of the serrations is not disturbed, it is evident that the motion must be in lines traced upon the curved surface of a body of air—exactly similar in size and form to one, two, three, etc., pulsations sent forth by the organ-pipe. Prof. Mayer was the first to trace the surface of sound-waves by this beautiful and ingenious method. It is highly probable that, by this arrangement, some hitherto unapproachable acoustic problems may meet with a solution.
The velocity of sound is not influenced by variations in the density of a uniform gaseous medium, provided the temperature of this medium remain stationary. But, when the temperature changes, the velocity is at once affected. Hence, a gradual rise in the temperature of the air, passing from the resonance-sphere to the gas-jet, will be productive of a successive alternation of coincidences and non-coincidences of serrated