THE ATMOSPHERE AND FOG-SIGNALING. 555 



8th of October, as already stated, the siren was established at the 

 South Foreland. I visited the station on that day, and listened to its 

 echoes. They were far more powerful than those of the horn. Like 

 the others, they were perfectly continuous, and faded, as if into dis- 

 tance, gradually away. The direct sound seemed rendered complex 

 and multitudinous by its echoes, which resembled a band of trump- 

 eters first responding close at hand, and then retreating rapidly 

 toward the coast of France. The siren-echoes on that day had 11 

 seconds', those of the horn 8 seconds' duration. 



In the case of the siren, moreover, the reenforcement of the direct 

 sound by its echo was distinct. About a second after the com- 

 mencement of the siren-blast, the echo struck in as a new sound. 

 This first echo, therefore, must have been flung back by a body of 

 air not more than 600 or 700 feet in thickness. The few detached 

 clouds visible at the time were many miles away, and could clearly 

 have had nothing to do with the effect. 



On the 10th of October, I was again at the Foreland, listening to 

 the echoes, with results similar to those just described. On the 15th 

 I had an opportunity of remarking something new concerning them 

 at Dungeness, where a horn, similar to, though not so powerful as, 

 those at the South Foreland, has been mounted. It rotates automati- 

 cally through an arc of 210, halting at four different points on the 

 arc and emitting a blast of 6 seconds' duration, these blasts being 

 separated from each other by intervals of silence of 20 seconds. 



The new point observed was this : As the horn rotated the echoes 

 were always returned along the line in which the axis of the horn 

 pointed. Standing either behind or in front of the light-house tower, 

 or closing the eyes so as to exclude all knowledge of the position of 

 the horn, the direction of its axis when it sounded oould always be 

 inferred from the direction in which the aerial echoes reached the 

 shore. Not only, therefore, is knowledge of direction given by a 

 sound, but it may also be given by the aerial echoes of the sound. 



On the 17th of October, at about 5 p. m., the air being perfectly 

 free from clouds, we rowed toward the Foreland, landed, and passed 

 over the sea-weed to the base of the cliff. As I reached the base, 

 the position of the Galatea was such that an echo of astonish- 

 ing intensity was sent back from her side ; it came as if from an inde- 

 pendent source of sound established on board the steamer. This 

 echo ceased suddenly, leaving the aerial echoes to die gradually into 

 silence. 



At the base of the cliff a series of concurrent observations made 

 the duration of the aerial siren-echoes from 13 to 14 seconds. 



Lying on the shingle under a projecting roof of chalk, the some- 

 what enfeebled diffracted sound reached me, and I was dble to hear 

 with great distinctness, about a second after the starting of the siren- 

 blast, the echoes striking in and reenforcing the direct sound. The 



