HECENT EXPERIMENTS ON FOG SIGNALS. 211 



mental yignificance. There was no visible reflecting sur- 

 face 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 fur 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 fur- 

 ther 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 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 contin- 

 uity 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 inten- 

 sity 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 

 degrees, 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 

 circumstances, come from the glassy sea; while both their 

 variation of direction and their perfectly continuous fall 

 into silence, are irreconcilable with the notion that they 

 carne from fixed objects on the land. They came from 

 that portion of the atmosphere into which the trumpet 

 poured its maximum sound, and fell in intensity as the 

 direct sound penetrated to greater atmospheric distances. 



The day on which our latest observations were made was 

 particularly fine. Before reaching Dungeness, the smooth- 

 ness of the sea and the serenity of the air caused me to 

 test the echoing power of the atmosphere. A single ship 

 lay about half a mile distant between us and the land. 

 The result of the proposed experiment was clearly foreseen. 



