MISCELLANY. 



379 



circles; the former are generated around 

 that point in concentric spheres ; all devia- 

 tions 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 cor- 

 responding to the periods of maximum 

 swelling (rarefaction) and contraction (con- 

 densation). 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 deline- 

 ate the march of sound within the wave. 



This result may be obtained approxi- 

 mately 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 impedi- 

 ment 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 du- 

 plicated. The resonance-spheres (resona- 

 tors) 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 de- 

 pending 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 trans- 

 mitted 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 man- 

 ner 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 sur- 

 face 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 co- 

 incidences and non-coincidences of serrated 



