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I HAVE been impressed for some time with the unsatisfactory character of the present fog signals. We must recognize that powerful siren signals are sometimes inaudible at distances but little exceeding a mile. It is true that these worst cases of inaudibility may not recur during fogs—as to this there seems to be insufficient evidence. But even when a sound-in-air signal is audible, the information conveyed is far from precise. The bearing of the source cannot be told with much accuracy, indeed some say that it cannot be told at all. The distance is still more uncertain. I should say that no system is satisfactory which does not give either the one or the other element, bearing or distance.
The system of synchronous signalling explained by Prof. Joly claims to give the distance with sufficient precision, and the American and Russian trials show that the claim is justified, as might indeed have been expected with some confidence, provided both signals themselves are well defined in time. The wireless electric signals are easily made sharp. Submarine signals from a bell, or explosive, would also be sharp enough. So probably would be explosive signals in air. The case of siren signals is more doubtful. Possibly the end might be sharp enough. Even so, the objection of the uncertain carrying of air signals remains.
I do not know whether there is already sufficient experience of submarine signals. If it be true that they can be depended upon up to distances of at least 4 or 5 miles, the case is strong for a combination of them with electric signals.
In some respects the system described in my former memorandum of 1916* has its advantages. It would give the bearing with electric signals only, but requires further experimenting, which if desired could be arranged for at the National Physical Laboratory but perhaps not during the war.
I am strongly of opinion that whatever is possible at the present time should be done to prepare the way for a better system.
"• This volume, p. 398.           •    •
R.VI.                                                   '                                          33ting to a single obstacle ; but if the distribution were in regular patterns, complications would ensue from the necessity for taking phases into account, as in the theory of gratings. For the present purpose it suffices to consider a random •distribution, although we may suppose that the centres, or more generally •corresponding points, of the obstacles lie in a plane perpendicular to the •direction of the primary light.