540 HISTORY OF SCIENCE. 



Some years afterwards an elegant instrument was invented by 

 CRAGNIARD DE LA TOUR. The Syren, for so the instrument was called 

 because it could sound with water as well as with air, is represented 

 in Fig. 257. A shows the external appearance of the syren; a is a 

 metallic disk perforated with a number of holes say eight at equal 

 distances. This disc is -attached to the spindle b, which turns with it 

 with the least possible friction. The disc covers as closely as may be 

 the fixed top of the drum c, consisting of a plate of metal perforated 

 with holes coinciding with those of the disc. These holes are not 

 perforations perpendicular to the surfaces of the plates ; they pass 

 obliquely through both, but the direction of the obliquity in the disc 

 is opposed to that in the fixed plate. At d this is shown in section, 

 and it will be seen that air escaping by the lower orifice as the upper 

 one passes over it will drive the disc in the direction of the arrow. 

 When, therefore, air is forced into the drum at <:, the effect will be 

 that when the rotation of the disc has once been set up, it will be 

 accelerated by the impulse of the escaping air until it attains a constant 

 velocity, determined by the pressure of the air mV. As the disc re- 

 volves, the passage of the air is alternately permitted and prevented, 

 and in this way sounds are produced. It is only necessary to count 

 the number of revolutions per second made by the disc while the 

 instrument emits a given note, to ascertain the number of impulses 

 corresponding with that note. The revolutions are registered on dials 

 by causing the teeth of a wheel to engage an endless screw on the 

 spindle. The buttons/^ are for throwing the counting apparatus into 

 and out of action at any required instant of time. The syren was 

 improved by the physicist Dove, who made the disc with several con- 

 centric circles of holes, increasing in number from the inside to the 

 outside, with a separate air-chamber to each circle, to which the air 

 was admitted at pleasure. By this arrangement the instrument could 

 be made to sound several different notes simultaneously. Further 

 improvements have since been introduced by Helmholtz. 



About 1830 M. FELIX SAVART, a skilful French experimenter, set 

 himself to investigate the limits of the sensibility of the ear. In his 

 researches he adopted a method of producing musical notes which 

 appears to have been first employed by that extraordinary genius Dr. 

 Hooke (page 220), who in 1681 exhibited before the Royal Society 

 a plan of producing musical notes by the striking of the teeth of wheels 

 in very rapid rotation. Savart's apparatus allowed him to increase at 

 will the sharpness of the shocks given to the air, and at the same time 

 to determine with the greatest precision the number of strokes or vibra- 

 tions per second. He used brass wheels of different sizes, from 9 to 

 30 inches in diameter, divided each into several hundred teeth. A 

 piece of card, or a thin strip of wood, was so fixed that each tooth 

 should strike it in passing. It was easy, by a simple mechanism, to 

 cause the number of rotations to be registered on a dial-plate. On 



