1874] WRITINGS OF JOSEPH HENRY. 385 



a pressure of from 20 to 35 pounds, while the trumpet was 

 sounded with a pressure of from 12 to 15 pounds. In the 

 case of the whistle, the pressure in the reservoir may be 

 indefinitely increased with an increase of the penetrating 

 power of the sound produced, while in the case of the trumpet 

 a pressure greater than a given amount entirely stops the 

 blast by preventing the recoil of the vibrating tongue; 

 this being made of steel, in the larger instruments 2^ inches 

 wide and 8 inches long, would receive a pressure of steam, 

 at onl}'- 10 pounds to the square inch, of 200 pounds, tending 

 to press it into the opening and to prevent its recoil, this 

 circumstance limits the power of a trumpet of given dimen- 

 sions. It is well fitted however to operate with a hot-air 

 engine, and is the least expensive in fuel of any of the 

 instruments now employed. The whistle is the simpler and 

 easier of management, although they both require arrange- 

 ment of machinery in order that they may be operated auto- 

 matically. 



It is a matter of much importance to obtain a hot-air 

 engine of sufficient power, and suitable for working fog-sig- 

 nals of all classes. This will be evident when we consider 

 the difficulty in many cases of obtaining fresh water for pro- 

 ducing steam, and the expense of the renewal of the boilers 

 in the use of salt-water, as well as that of the loss of power 

 in frequently blowing out the latter, in addition to the dan- 

 ger of the use of steam by unskillful attendants. 



The merits of the two engines however under considera- 

 tion could not be fully tested by the short trial to which 

 they were subjected during these experiments. The princi- 

 pal objection to the Ericsson engine was the size of the fly- 

 wheel and the weight of the several parts of the machine ; 

 the Roper engine was much more compact, and appeared to 

 work with more facility, but from the greater heat imparted 

 to the air the packing was liable to burn out and required 

 to be frequently renewed. Although at first the impression 

 of the committee was in favor of the Roper engine, yet in 

 subsequent trials of actual practice it was found too difficult 

 to be kept in order to be employed for light-house purposes, 

 25 



