394 WRITINGS OF JOSEPH HENRY. [1874 



brief intervals allowed to escape. Immediately before this 

 head, and in close contact with it, is a revolving disk in 

 which are eight orifices. By this arrangement, at every 

 complete revolution of the disk, the orifice in the head of 

 the drum is opened and shut eight times in succession, thus 

 producing a rapid series of impulses of steam against the 

 air into the smaller orifice of the trumpet placed immedi- 

 ately in front of the revolving disk. These impulses are of 

 such energy and rapidity as to produce a sound unrivalled 

 in intensity and penetrating power by that of any other 

 instrument yet devised. 



The siren was operated by an upright cylindrical tubular 

 boiler, with a pressure of from 50 to 100 pounds on the 

 square inch. This form of boiler was subsequently replaced 

 by an ordinary horizontal locomotive-boiler w^ith a small 

 engine attached for feeding it, and for rotating the disk, the 

 latter being effected by means of a band passing over pulleys 

 of suitable relative dimensions. 



3d. A steam-whistle 8 inches in diameter. Through some 

 mis-understanding a series of whistles of different diameters 

 was not furnished as was intended. 



The first experiments to be noted were those in regard to a 

 comparison of the penetrating power of the siren and the 

 whistle; the fitting up of the DaboU trumpet not having been 

 completed. The principal object of this however, was to test 

 again the truthfulness of the indications of the artificial ear 

 in comparison with those of the natural ear. 



An experiment was made both by means of the artificial 

 ear on land, and by actually going off on the ocean in a 

 steamer until the sounds became inaudible to the natural 

 ear. By the latter method the two sounds ceased to be heard 

 at the distances of six and twelve and a half miles, respec- 

 tively. The indications of the artificial ear gave a similar 

 result, the distance at which the sand ceased to move in one 

 case being double that of the other. In both cases the con- 

 ditions of wind and weather were apparently the same. In 

 the case of the steamer the distance was estimated by noting 

 the interval of time between the flash of steam and the per- 

 ception of the sound. 



