“78 PHYSICS. 
sented in pl. 19, figs. 589—61, where ¢,t', f, f’, is a round box of brass about 
two or three inches broad and one inch high, whose upper surface is perfectly 
plane and well polished ; there is an opening in the middle of the bottom ff’, 
into which the air tube, gg’,is screwed. In the bottom, tt’, represented from 
above, and laterally in fig. 60, a number of holes equidistant from each 
other are bored, their interspaces being somewhat greater than the diameter 
of the holes, which generally amount to ten ; pp’ is a movable plate, ground 
upon the plate ¢é', and provided with holes corresponding in size, number, 
and position, with tt’, so that by turning pp’ about its axis, 2, on tt’, all the 
holes may be simultaneously opened or closed. At the upper extremity of the 
axis x there is an endless screw, catching in a wheel, 77”, of 100 teeth; ec’ 
is a second wheel of 100 teeth, standing in such connexion with the first 
that it completes only one revolution while the first makes 100, an arm on 
the axis of the first wheel pushing the second forwards by one tooth at each 
revolution. ‘The axes of these two wheels carry indices, which mark on the 
dials attached to the side plate (as represented in fig. 61) the revolutions and 
their fractions. To start this part of the machinery, or arrest its motion at 
any moment, the axis of the wheel v7’ is united in such a manner with the 
buttons 6 and 0’, that this wheel can either be caught in the endless screw 
or separated from it. The apertures in the plates tt’ and pp’ are directed 
obliquely to the surface, so that the air rushing through gg’ is capable of 
causing a rapid rotation of the plate pp’. Suppose, now, that in the movable 
disk there are ten holes, and in the other only one, then this would be 
opened and shut ten times in a revolution of the plate: there thus arise ten 
complete sound waves in one revolution, of which there may be 1, 10, 100, 
&c., in a second, so that all the tones may thus be produced. The lower 
plate has, however, ten holes; and as each one exerts its influence, there is 
produced a strong lasting tone. 
To count vibrations with this instrument (called by its inventor the 
Siren), place upon the sound-board (fig. 58) a concordant pipe, as the a of 
the common tuning fork, and near it the siren in another hole of the sound 
board. Allow the air to enter, and regulate the pressure upon the wind box 
by the rod ¢, until the two are in unison; then couple the wheel of the siren, 
and allow it to revolve a certain time by a seconds-watch. Stop the 
motion of both watch and siren, and from the latter may be obtained the 
entire number of revolutions, and from the former the number of seconds ; 
comparing the two will give us the number per second. We shall then find 
that in one second 440 revolutions have been made, which is really the num- 
ler of vibrations for the tone a of the tuning fork. 
he vibrations of strings are much too rapid to admit of their being 
counted ; they are even visible only in the longest and deepest strings. It 
was known very early that the tone of a string was higher the more the 
string was stretched, or when it was shortened. It was not possible, how- 
ever, to indicate by means of calculation the connexion between the tone 
of a string, its tension, its length, and the rapidity of its vibration. The 
eminent philosophers, Taylor, the two Bernouillis, d'Alembert, and Euler, 
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