Page:Popular Science Monthly Volume 6.djvu/573

Rh 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 distance, gradually away. The direct sound seemed rendered complex and multitudinous by its echoes, which resembled a band of trumpeters 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 reënforcement of the direct sound by its echo was distinct. About a second after the commencement 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 automatically 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 could always be inferred from the direction in which the aërial 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, 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 astonishing intensity was sent back from her side; it came as if from an independent source of sound established on board the steamer. This echo ceased suddenly, leaving the aërial 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 somewhat enfeebled diffracted sound reached me, and I was able to hear with great distinctness, about a second after the starting of the siren-blast, the echoes striking in and reënforcing the direct sound. The