Treatise on human physiology : for the use of students and practitioners of medicine / by Henry C. Chapman.

  • Henry Cadwalader Chapman
Date:
1887
Catalogue details

Licence: Public Domain Mark

Credit: Treatise on human physiology : for the use of students and practitioners of medicine / by Henry C. Chapman. Source: Wellcome Collection.

    818/942 page 830
    Do*, is due, as before, to the rate of vibration being 256 per second, the other Do? an octave below, the rate being just half, or 128 vibrations per second, the air issuing from 8 orifices during one rotation of the disk, instead of from 16, as in the former case. The innermost and outer- most series of orifices being open, then the rate of vibration of the two sounds is as 1 to 2. From what has just been said, it is evident, without further explanation, that if the innermost series of 8 orifices and the next series to it of 10 orifices be open, then the rate of vibration will be as 4 to 5—that is to say, of the two notes produced by the siren simultaneously, if one be Do’, due to 128 vibrations per second, the other note will be Mi?’ E?, due to 160 vibrations per second, and if the innermost series of 8 orifices and the third series from within outward, that of 12 orifices, be both open, then the rate of vibration of the two sounds will be as 2 to 3—that is, if Do? be one of the two notes heard simultaneously the other note will be Sol’ G’, due to 198 vibrations per second. It follows, therefore, that if the three innermost series of orifices, 8, 10, and 12, be open we will obtain the major chord Do? E? G? Do? M? Sol’, and if in addition at the same time the outermost series of 16 orifices be open C? E? G? C®, which, as produced by the siren, is quite musical. The construction and manner of using a single Dove siren being now understood, there will be no difficulty in comprehending that of Helmholtz, it consisting, as already mentioned, of two dove sirens. It must be mentioned, however, that the outermost series of orifices in the lower of the two sirens are 18 in number instead of 16, as in the single Dove siren we have just described, and that the orifices in the four series of the upper siren are respectively 9, 12, 15, and 16 in number. If, therefore, we wish to obtain by the double siren the major chord C’ EK? G* and C* at the same time, the series of orifices 8, 10, and 12 must be opened in the lower siren, and the series of 16 orifices of the upper siren, the air being allowed, of course, to pass from the bellows through both sirens, the number of rotations of the disks being recorded in the same manner as already described by wheel-work attached to the axis common to the two sirens, and omitted for simplicity in Fig. 499. It will be observed that the double siren is provided with resonance boxes the half of each of which has been removed in the figure, which greatly intensifies the fundamental sound. The upper siren is also connected with a toothed wheel and pinion turned by a handle, by which we are enabled to rotate the cylinder of the upper siren as well as the disk, the handle, etc., being so disposed that if it be turned to the right the orifices in the cylinder and in the disk will then pass over each other more quickly than if the cylinder was at rest, and if in the reverse direction more slowly, the pitch rising in the one case, the puffs succeed- ing each other then more rapidly, and falling in the other, following more slowly. ‘The use of the dial and index underneath the handle will be illustrated hereafter. It is needless to Bay that by doubling all parts of the siren, etc., as done by Helmholtz,' that many varied com- binations can be “introduced, and that the general usefulness of the instrument has thereby been greatly increased. 1 On the Sensation of Tone, trans], by A. J, Ellis, p. 245. London, 1875.
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