RESEARCHES IN SOUND. 473 



along this distance was exceedingly difficult, and, to obviate tliis, a car- 

 riage with broad wheels, drawn by two horses, was employed. An 

 awning over this vehicle protected the observer from the sun, and ena- 

 bled him, without fixtigue and at his ease, to note the agitations of 

 sand on the drum of the artificial ear, the mouth of which was directed 

 from the rear of the carriage toward the sounding instrument. 



For these and other facilities we were indebted to General Humphreys, 

 Chief of the Engineer Bureau, who gave orders to the officer in charge 

 of the military works at Sandy Hook to afford us every aid in his power 

 in carrying on the investigation. 



The instruments employed were — 



1st. A first-class Daboll trumpet (the patent for which — since the 

 death of Mr. Daboll, is owned by Mr. James A. Eobinson,) operated by 

 an Ericsson hot-air engine. It carried a steel reed 10 inches long, 2^ inches 

 wide, and \ inch in thickness at the vibrating end, but increasing gradu- 

 ally to an inch at the larger extremify. This was attached to a large 

 vertical trumpet curved at the upi)er end into a horizontal direction and 

 furnished with an automatic arrangement for producing an oscillation 

 of the instrument of about G0° in the arc of the horizon. Its entire 

 length, including the curvature, was 17 feet. It was 3J inches at the 

 smaller end and had a ilaring mouth 38 inches in diameter. The engine 

 had a cylinder 32 inches in diameter, with an air-chamber of 4J feet in 

 diameter and C feet long, and was able to furnish continually a five-second 

 blast every minute at a pressure of from 15 to 30 i^ounds. 



2d. A siren, originally invented by Cagniard de Latour, and well 

 known to the physicist as a means of comparing sounds and measuring 

 the number of vibrations m different musical notes. Under the direc- 

 tion of the Light-House Board, Mr. Brown, of Xew York, had made a 

 series of experiments on this instrument in reference to its adoption as 

 a fog-signal, and these experiments have been eminently successful. 

 The instrument as it now exists differs in two essential liarticulars 

 from the original invention of Latour: 1st, it is connected with a 

 trumpet in which it supplies the place of the reed in producing the agi- 

 tation of the air necessary to the generation of the sound ; and 2d, the 

 revolving disk, which opens and shuts the orifices producing the blasts, 

 is driven not by the blast itself impinging on oblique openings, as in 

 the original instrument, but by a small engine connected with the feed- 

 pump of the boiler. 



The general character of the instrument may be understood from the 

 following description : Suppose a drum of short axis, into one head of 

 which is inserted a steam-pij^e connected with a locomotive-boiler, while 

 the other end has in it a triangular orifice, through which the steam is 

 at brief intervals allowed to project itself. Immediately before tliis 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 



