2 Prof. J. Joly on Scientific 



" The Mariner should not assume — 



a. That because he fails to hear the sound he is out of 



hearing distance. 



b. That, because he hears a fog-signal faintly, he is at a 



great distance from it. 



c. That because he hears the sound plainly he is near it. 



d. That, because he does not hear it, even when in close 



proximity, the fog-signal has ceased sounding. 



e. That the distance from and the intensity of the «ound 



on any one occasion, are a guide to him for any future 

 occasion. 



Taken together, these facts should induce the utmost caution 

 in closing the land in fogs. The lead is generally the onlv 

 sate guide. 



It would be, of course, entirely wrong to conclude that 

 such drastic warnings are intended to imply the general 

 worthlessness of aerial sound signals. It is probable that 

 the disuse of such signals would not find favour. Our 

 present purpose is rather to consider additional aids to 

 navigation whereby the sailor escapes the special dangers 

 arising from the failure of aerial fog-signals, and is supplied 

 with other signals at once more reliable, heard at greater 

 •distances, and giving him information beyond the power of 

 aerial fog-signals to convey. Such modern methods of 

 signalling are based on recent advances in science. 



We shall consider first what may be called " synchronous 

 signalling,'' that is the use of signals propagated in different 

 media but timed so as to start at the same instant. 



The principles of synchronous signalling have for long- 

 formed a part of familiar household science. When timid 

 people see the flash of lightning and hear the crash of thunder 

 they feel reassured when they perceive an appreciable 

 interval separating the one phenomenon from the other. 

 On the other hand, when both occur together they infer, and 

 rightly so, that there is more danger. And most people are 

 aware of the principle underlying this inference. If the 

 seat of the electric discharge, the flash itself, in fact, is 

 remote, the sound originated by it, i. e. the thunder, takes an 

 appreciable time to reach the ear. Travelling nearly 1100 

 feet in a second, this time interval may amount to several 

 seconds. On the other hand, the velocity of propagation of 

 light is so enormous that we may consider that we see the 

 flash at the very instant of its occurrence however remote it 

 may be placed. Hence if one second intervenes between 

 the moment of seeing the flicker of the lightning and hearing 



