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84                      HAND-BOOK OF ACOUSTICS.
Although this instrument does very well for purposes of lecture illustration, it has several practical defects. When, for instance, the registering apparatus is thrown into gear, the increased work which the wind has to perform in turning the cogwheels, slackens the speed, and consequently lowers the pitch. Then, again, it is very difficult to keep the blast at a constant pressure—that is, to keep the sound steady. Further, there is an opening for error in noting the exact time of opening and closing the recording apparatus. From these causes, this form of Syren cannot be depended upon, according to Mr. Ellis, within ten vibrations per second.
Helmholtz has devised a form of Syren, in which these sources of error are avoided. It consists really of two Syrens, A and B, fig. 22, facing one another, the discs of which are both mounted on the same axis, and driven with uniform velocity, not by the pressure of the wind, but by an electro-motor. Each disc has 4 circles of equidistant holes, the number of holes in the circles being, in the lower disc 8, 10, 12, 18, and in the upper 9, 12, 15, 16 respectively. By means of the handles 1, 2, 3, &c, projecting from the wind chests, any or all of these circles may be closed or opened, so that two or any number of sounds up to eight, may be heard simultaneously.
Knowing the length of a stretched string, the stretching weight, and the weight of the string itself, it is possible to ascertain the vibration number of the sound it gives by calculation. The instrument used in applying this method, is termed a Sonometer
Fig. 23.
or Monochord. It consists (fig. 23) of a sound box of wood, about 5 feet long, 7 inches broad, and b inches deep. A steel wire is fastened to a peg at one end, passes over bridges as shown