THE ATMOSPHERE AND FOG-SIGNALING. 703 



tions have been so numerous and long-continued as to enable us to come 

 to the sure conclusion that, on the whole, the steam-siren is the most 

 powerful fog-signal which has hitherto been tried in England. It is 

 specially powerful when local noises, such as those of wind, rigging, 

 breaking waves, shore-surf, and the rattle of pebbles, have to be over- 

 come. Its density, quality, pitch, and penetration, render it dominant 

 over such noises after all other signal-sounds have succumbed. 



I have not, therefore, hesitated to recommend the introduction of 

 the siren as a coast-signal. 



It will be desirable in each case to confer upon the instrument a 

 power of rotation, so as to enable the person in charge of it to point 

 its trumpet against the wind or in any other required direction. This 

 arrangement was made at the South Foreland, and it presents no 

 mechanical difficulty. It is also desirable to mount the siren so as 

 to permit of the depression of its trumpet fifteen or twenty degrees 

 below the horizon. 



In selecting the position at which a fog-signal is to be mounted, the 

 possible influence of a sound-shadow, and the possible extinction of the 

 sound by the interference of the direct waves with waves reflected 

 from the shore, must form the subject of the gravest consideration. 

 Preliminary trials may, in most cases, be necessary before fixing on 

 the precise point at which the instrument is to be placed. 



The siren, it will be remembered, has been hitherto worked with 

 steam of TO lbs. pressure or thereabouts ; the trumpets have been 

 worked with compressed air ; and our experiments have proved that a 

 pressure of 20 lbs. yielded sensibly as loud a sound as higher press- 

 ures. The possibility of obtaining a serviceable sound with this low 

 air-pressure may render available the employment of caloric engines 

 with trumpets; if so, the establishment of trumpets on board light- 

 vessels would be greatly facilitated. The signals at present existing 

 on board such vessels are exceedingly defective, and may be immeas- 

 urably improved upon. There are, I am told, practical difficulties as 

 to the introduction of steam on board light-ships ; otherwise I should 

 be strongly inclined to recommend the introduction among them of 

 the Canadian whistle. The siren would probably be found too large 

 and cumbrous for light-vessels. 



The siren, which has been long known to scientific men, is worked 

 with air, and it would be worth while to try how the fog-siren would 

 behave supposing compressed air to be substituted for steam. Com- 

 pressed air might also be tried with the whistles. 



No fog signal hitherto tried is able to fulfill the condition laid down 

 in a very able letter already referred to, namely, that " all fog-signals 

 should be distinctly audible for at least 4 miles, under every circum- 

 stance." Circumstances may exist to prevent the most powerful sound 

 from being heard at half this distance. What may with certainty be 

 affirmed is, that in almost all cases the siren may certainly be relied on 



