Text: 7) Knowing the velocity of sound in air we can estimate the different wave lengths corresponding to notes of different pitch in the following manner. The wave length is the distance through which the sound travels while any particle over which it passes describes a complete vibration; hence, if we know the number of vibrations the particle performs per second, the required wave length will be found by dividing the number of feet over which the sound travels per second, by that number. Now, by means of an instrument invented by Cagniard de la Tour, and by him named syren, the number of vibrations corresponding to a note of any given pitch can be determined very exactly. For a detailed account of this instrument and of its improvements by Helmholtz, the reader is referred to Tyndall's Lectures on Sound, p. 64; but to describe it shortly may be said in its original form to consist of two equal disks, one forming the top of a hollow fixed cylinder, into which air can be driven, the other capable of revolving concentrically upon it with the smallest possible amount of friction. A circle of small holes equidistant from each other is bored upon each disk and concentric with it; those in the upper disk being inclined slantwise to its plane, those in the lower being slantwise also but in the opposite direction; there are also arrangements both for driving a constant supply of air into the hollow cylinder, and for regestering the number of revolutions the upper disk performs in a minute; thus, when the upper disk is so turned that its holes coincide with those of the lower, and air is forced into the cylinder, it will pass out through the perforations, and by reason of their obliquity will cause the moveable disk to revolve with a rapidity corresponding to the pressure; and each time that the holes of the former coincide with those of the latter a number of little puffs of air get through simultaneously, giving rise to an agitation in the surrounding atmosphere which spreads round in all directions in the way before described, and if the pressure of the air in the cylinder is sufficient, the series of impulses thus given will link themselves together, forming a continuous note. (It should be remarked that the pitch of the sound would be exactly the same if there were only one perforation in the revolving disk, the number of holes merely serving to increase its intensity; if the number of holes in the revolving disk is less than the number in the lower one, those of the former must be situated so as all to coincide simultaneously with an equal number of the latter. (It should also be remarked that this syren is an old invention. Today there are any number of electronic and computer instruments that are far more accurate and precise than this mechanial device.) Hence to determine the number of vibrations per second, corresponding to a sound of a given pitch, we have only to maintain such a pressure of air in the syren as will cause it to produce the same sound for the space of a minute, and note the number of revolutions registered in that time. Now, for every revolution of the upper disk, the same number of sound waves are propagated around as there are perforations, hence the whole number propagated in a second will be the product of the number of holes and number of revolutions per minute divided by 60; and this result will evidently be the required number of vibrations per second caused by the given sound. To apply this to find the wave length corresponding to the note given by the open C string of the violoncello, we should adjust the supply of air to the syren until it gives a note of the same pitch. Supposing the number of holes in each disk to be 18, the number of revolutions per minute would be found to be 220. Hence the number of vibrations per second of the string, and therefore of the surrounding particles of air, would be 220 x 18/ 60 = 66. Supposing the temperature were 16° C the velocity of sound would be about 1122 feet per second, and the number by 66 gives the wave length correspnding to that number of vibrations per second; that is, just 17 feet; the sound then will travel through this distance during the time the string takes to perform one complete vibration.

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