RECENT EXPERIMENTS ON FOG-SIGNALS. 275 



of having an organized code of fog-signals for the fleet. 

 He shook his head doubtingly, and referred to the diffi- 

 culty of finding room for signal guns. The gun-cotton 

 rocket completely surmounts this difficulty. It is 

 manipulated with ease and rapidity, while its dis- 

 charges 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 es- 

 tablishment 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 

 Report on Fog-signals, addressed to the Corporation of 

 Trinity House in 1874.* To these echoes I attached 

 a fundamental significance. There was no visible re- 

 flecting 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 duration, 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 fur- 

 ther 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, ' taper- 

 ing ' by imperceptible gradations into absolute silence. 

 But when a ship happened to. throw itself athwart the 



* See also ' Philosophical Transactions' for 1874, p. 188. 



