556 THE POPULAR SCIENCE MONTHLY. 



first rush of echoed sound was very powerful, and it came, as usual, 

 from a stratum of air 600 or 700 feet in thickness. On again testing 

 the duration of the echoes, it was found to be from 14 to 15 seconds. 

 The perfect clearness of the afternoon caused me to choose it for the 

 examination of the echoes. It was worth remarking that this was our 

 day of longest echoes, and it was also our day of greatest acoustic 

 transparency, this association suggesting that the duration of the 

 echo is a measure of the atmospheric depths from which it comes. On 

 no day, it is to be remembered, was the atmosphere free from invisi- 

 ble acoustic clouds ; and on this day, and when their presence did not 

 prevent the direct sound from reaching to a distance of 15 or 16 nau- 

 tical miles, they were able to send us echoes of 15 seconds' duration. 



On various occasions, when fully three miles from the shore, the 

 Foreland bearing north, we have had the distinct echoes of the siren 

 sent back to us from the cloudless southern air. 



To sum up this question of aerial echoes. The siren sounded 

 three blasts a minute, each of 5 seconds' duration. From the num- 

 ber of days and the number of hours per day during which the in- 

 strument was in action, we can infer the number of blasts. They 

 reached nearly 20,000. The blasts of the horns exceeded this num- 

 ber, while hundreds of shots were fired from the guns. Whatever 

 might be the state of the weather, cloudy or serene, stormy or calm, 

 the aerial echoes, though varying in strength and duration from day 

 to day, were never absent; and on many days, "under a perfectly 

 clear sky," they reached, in the case of the siren, an astonishing in- 

 tensity. It is to these air-echoes and not to cloud-echoes, that the 

 rolling; of thunder is to be ascribed. 



'& 



Experimental Demonstration of Aerial Reflection. Thus far 

 we have dealt in inference merely, for the interception of sound 

 through aerial reflection has never been experimentally demonstrated ; 

 and, indeed, according to Arago's observation, which has hitherto 

 held undisputed possession of the scientific field, it does not sensibly 

 exist. But the strength of science consists in verification, and I was 

 anxious to submit the question of aerial reflection to an experimental 

 test. As in most similar cases, it was not the simplest combinations 

 that were first adopted. Two gases of different densities were to be 

 chosen, and I chose carbonic acid and coal gas. With the aid of my 

 skillful assistant, Mr. John Cottrell, a tunnel was formed, across which 

 five-and-twenty layers of carbonic acid were permitted to fall, and 

 five-and-twenty alternate layers of coal-gas to rise. Sound was sent 

 through this tunnel, making fifty passages from medium to medium 

 in its course. These, I thought, would waste in aerial echoes a sensi- 

 ble portion of sound. 



To indicate this waste an objective test was found in a gas-flame 

 brought to the verge of flaring. The action of sonorous vibrations on 



