MEMORANDUM ON FOG SIGNALS. [Report to Trinity House, May 1916.]
PROLONG-ED experience seems to show that, no matter how much powc may be employed in the production of sound-in-air signals, their audibilit cannot be relied upon much beyond a mile. At a less distance than t\v miles the most powerful signals may be lost in certain directions when tl: atmospheric conditions are unfavourable. There is every reason to surmis that in these circumstances the sound goes over the head of the observer, bu so far as I know, there is little direct confirmation of this. It would clear u the question very much could it be proved that when a signal is premature! lost at the surface of the sea it could still be heard by an observer at a coi siderable elevation. In these days of airships it might be possible to get decision.
But for practical purposes the not infrequent failure of sound-in-air signa must be admitted to be without remedy, and the question arises what alte natives are open. I am not well informed as to the success or otherwise submarine signals, viz. of sounds propagated through water, over long distance What I wish at present to draw attention to is the probable advantage of s< called "wireless" signals. The waves constituting these signals are indec for the most part propagated through air, but they are far more near independent of atmospheric conditions—temperature and wind—than a ordinary sound waves. With very moderate appliances they can be sent ar observed with certainty at distances such as 10 or 20 miles.
As to how they should be employed, it may be remarked that the me reception of a signal is in itself of no use. The signal must give informatix as to the distance, or bearing, or both, of the sending station. The estimatii of distance would depend upon the intensity of the signals received and wou probably present difficulties if any sort of precision was aimed at. On tl other hand the bearing of the sending station can be determined at t' receiving station with fair accuracy, that is to within two or three degrei The special apparatus required is not complicated, "but it is rather cumbro since coils of large area have to be capable of rotation. ' I assume that tl.......................... (6)p. 87 (1904) ; Selent^e Papers, Vol. v. p. 141).On account of the magnitude of x we have only the one curvature to deal with. For this curvature