256 Prof. Tyndall. Experiments on Fog- Signals. [Mar. 21. 



House in 1874.* 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 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 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 towards 

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

 stances, come from the glassy sea ; while both their variation of 

 direction, and their perfectly continuous fall into silence, are irrecon- 

 cileable with the notion that they came 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 particu- 

 larly fine. Before reaching Dungeness, the smoothness 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. It was this. The rocket being sent up, it exploded at a 

 great height; the echoes retreated in their usual fashion, becoming 

 less and less intense as the distance of the surfaces of reflection from 

 the observers increased. About five seconds after the explosion, a 

 single loud shock was sent back to us from the side of the vessel 

 lying between us and the land. Obliterated for a moment by this 



* See also " Philosophical Transactions" for 1874, p. 183. 



