SYREN, an instrument invented by M. Cagniard de la Tour for determining the number of vibrations per second of any given note.
The fanciful name of the instrument was given from the fact that it is capable of exciting vibrations in water or any other fluid. It con sists of a cylindrical chamber of brass c, about 3 inches in diameter, and 14 inch high, connected with which is a tube n, which fits into the tube of a double pair of bellows. The upper surface of c contains a circle of holes, inclined somewhat obliquely, and coinciding with this circle is another circle of holes, inclined at an angle with the first, and contained in a disc of metal h, free to move on is vertical axis. This axis has near its upper extremity a perpetual screw which gives motion to a wheel u, of 100 teeth, and this acts on a hundreds wheel, marked II, and this again on a thousands wheel, marked Y. Now suppose each circle of holes to contain 25. On sending a blast of air through H, it will, in escaping through the holes in c, cause the disc h to rotate with a speed depending on the force of the blast, and during each turn of the disc the currents will Went off and re-opened 25 times, producing in fact, 25 waves of sound. One hole in each disc would produce,*with a given velocity, the same note as 25, but the larger number produces a louder tone. By regulating the force of the
blast, the disc may be made to vary in speed, so as to produce notes from the gravest ;to the most acute. By making the syren give the note, the velocity of whose vibrations we wish to determine (that of a tuning fork for example), the total number of rotations of the disc, as recorded by the dials attached to the wheels u, rt, T, divided by the number of seconds during which the observation is being made, will give the number of vibrations per second for the note in question. By a simple adjustment, the endless screw can be put in and out of gear with the teeth of the wheels in a moment, so that the registration need not begin until the instrument is sounding the note fully and freely. A seconds pendulum should also be at hand, boating audibly, so that the observer may begin to count the moment the screw is put in gear. Some practice is required to make the instrument sustain the note for 8 or 10 seconds. Graduated dial-plates and moving hands, as In a gas-meter, are connected with the wheels u, u, T, so that the number of vibrations can be read off by simple inspection.