Prolonged experience seems to show that, no matter how much power may be employed in the production of sound-in-air signals, their audibility cannot be relied upon much beyond a mile. At a less distance than two miles the most powerful signals may be lost in certain directions when the atmospheric conditions are unfavourable. There is every reason to surmise that in these circumstances the sound goes over the head of the observer, but, so far as I know, there is little direct confirmation of this. It would clear up the question very much could it be proved that when a signal is prematurely lost at the surface of the sea it could still be heard by an observer at a considerable elevation. In these days of airships it might be possible to get a decision.

But for practical purposes the not infrequent failure of sound-in-air signals must be admitted to be without remedy, and the question arises what alternatives are open. I am not well informed as to the success or otherwise of submarine signals, viz. of sounds propagated through water, over long distances. What I wish at present to draw attention to is the probable advantage of socalled “wireless” signals. The waves constituting these signals are indeed for the most part propagated through air, but they are far more nearly independent of atmospheric conditions—temperature and wind—than are ordinary sound waves. With very moderate appliances they can be sent and observed with certainty at distances such as 10 or 20 miles.

As to how they should be employed, it may be remarked that the mere reception of a signal is in itself of no use.