384 WRITINGS OF JOSEPH HENRY, [1874 



to the introduction of this modification of the apparatus into 

 light-ships. 



The other instruments which were subjected to trial were 

 an ordinary steam-whistle and a Daboll trumpet. The bell 

 of the whistle was 6 inches in diameter, 9 inches in height, 

 and received the sheet of steam through an opening of one- 

 thirtieth of an inch in width ; was worked by a pressure of 

 condensed air of from 20 to 35 pounds per square inch, and 

 blown once in a minute for about five seconds. The air was 

 condensed by a Roper engine of one-horse power. The pen- 

 etrating power of the sound was increased by an increase in 

 the pressure of the air, and also the pitch. The tone how- 

 ever of the instrument was lowered by increasing the distance 

 between the orifice through which the circular sheet of air 

 issued at the lower rim of the bell or resounding cavity. 

 To prove conclusively that the bell performs the part of a 

 mere resounding cavity, a wooden one — on a subsequent 

 occasion, was substituted for that of metal without a change 

 in the loudness or the pitch of the sound. 



The penetrating power of the whistle was compared with a 

 Daboll trumpet, actuated by an Ericsson engine of about the 

 same power; the reservoir for the condensed air of each 

 machine was furnished with a pressure-guage, and by know- 

 ing the capacity of the condensing pumps and the number 

 of strokes required to produce the pressure, the relative 

 amount of power was determined. The result was that the 

 penetrating power of the trumpet was nearly double that of 

 the whistle, and that an equal effect was produced at the 

 same distance by about one-fourth of the power expended 

 in the case of the latter. It must be recollected though that 

 the whistle sends sonorous waves of equal intensity in every 

 direction, while the greatest power of the trumpet is in the 

 direction of its axis. This difference however is lessened on 

 account of the spreading of the sound to which we have be- 

 fore alluded.* The whistle was blown, as we have said, with 



*It is worthy of note however that in the case of a sound having primarily 

 an axial direction, the subsequent lateral diffusion must result in enfeebling 

 the whole sphere of expanding sound-waves in a more rapid ratio than the 

 square of the distance. 



