EECENT EXPERIMENTS ON FOG-SIGNALS. 327 



culty. It is manipulated with ease and rapidity, while 

 its discharges may be so grouped and combined as to 

 give a most important extension to the voice of the 

 admiral in command. It is needless to add that at any 

 point upon our coasts, or upon any other coast, where 

 its establishment might be desirable, a fog-signal 

 station might be extemporised without difficulty. 



I have referred more than once to the train of echoes 

 which accompanied the explosion of gun-cotton in free 

 air, speaking of them as similar in all respects to those 

 which were described for the first time in my Eeport on 

 Fog-signals, addressed to the Corporation of Trinity 

 House in 1874. 1 To these echoes I attached a funda- 

 mental significance. There was no visible reflecting 

 surface from which they could come. On some days, 

 with hardly a cloud in the air and hardly a ripple on 

 the sea, they reached a magical intensity. As far as the 

 sense of hearing could judge, they came from the body 

 of the air in front of the great trumpet which produced 

 them. The trumpet blasts were five seconds in dura- 

 tion, but long before the blast had ceased the echoes 

 struck in, adding their strength to the primitive note 

 of the trumpet. After the blast had ended the echoes 

 continued, retreating further and further from the point 

 of observation, and finally dying away at great distances. 

 The echoes were perfectly continuous as long as the sea 

 was clear of ships, ' tapering ' by imperceptible grada- 

 tions into absolute silence. But when a ship happened 

 to throw itself athwart the course of the sound, the echo 

 from the broadside of the vessel was returned as a shock 

 which rudely interrupted the continuity of the dying 

 atmospheric music. 



1 See also Philosophical Transactions ' for 1874, p. 183. 



