286 THE POPULAR SCIENCE MONTHLY. 



the Deputy Master of the Trinity House. Thanks to the skillful aid 

 given by the authorities of Woolwich, by Mr. Prentice and Mr. Brock 

 that idea is now an accomplished fact ; a signal of great power, handi- 

 ness, and economy, being thus placed at the service of our mariners. 

 Not only may the rocket be applied in association with lighthouses and 

 light-ships, but in the navy also it may be turned to important account. 

 Soon after the loss of the Vanguard, I ventured to urge upon an emi- 

 nent naval officer the desirability of having an organized code of fog- 

 signals for the fleet. He shook his head doubtingly, and referred to 

 the difficulty of finding room for signal-guns. The gun-cotton rocket 

 completely surmounts this difficulty. 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 sta- 

 tion might be extemporized without difficulty. 



I have referred more than once to the train of echoes which accom- 

 panied 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. 1 To these echoes I attached a fundamental signifi- 

 cance. 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 us with magical intensity. They came 

 directly 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 farther and farther 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 gradations 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. 



These echoes have been ascribed to reflection from the crests of the 

 sea-waves. But this hypothesis is negatived by the fact that the echoes 

 were produced in great intensity and duration when no waves existed 

 when the sea, in fact, was of glassy smoothness. It has been also 

 shown that the direction of the echoes depended not on that of waves, 

 real or assumed, but on the direction of the axis of the trumpet. 

 Causing that axis to traverse an arc of 210, and the trumpet to sound 

 at various points of the arc, the echoes were always, at all events in 

 calm weather, returned from that portion of the atmosphere toward 

 which the trumpet was directed. They could not, under the circum- 



1 Sec also "Philosophical Transactions" for 1874, p. 183. 



