On a method of avoiding collision at sea

I have in the foregoing paper already described a method of finding distances at sea in fog or thick weather. It is desirable to briefly recapitulate here the principle involved. signals travelling at differing rates are simultaneously sent out from the lighthouse or signal station. Thus there might he a sub-marine sound along with an aërial; or, again, a wireless signal along with an aërial sound; or, finally, the combination might he the sound in water and the wireless signal. Such simultaneously emitted signals become relatively displaced as they are propagated outwards. After travelling a distance of one nautical mile a sound in air will lag behind a sound in water by a time interval amounting to about 4.3 seconds. And an aërial sound will lag behind a wireless or a luminous signal as much as 5.5 seconds in the mile. Hence as the lag continues to increase at these rates per mile, the observation of the interval separating the reception of the signals on the ship will enable the navigator to "determine his distance from the signal station. It is obvious that similar methods will enable the mariner to tell the distance of another vessel, An error so great as one half second corresponds with an error of only 90 fathoms in the ease of the most sensitive combination—that is wireless signal and aërial sound.