Page:Popular Science Monthly Volume 6.djvu/723

Rh 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 overcome. 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 70 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 pressures. 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 immeasurably 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. Compressed 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 circumstance." 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