RESEAECHES IN SOUND. 553 



tlirong:li an atmosphere of comparative nniformitj' of temi)eratiire, or 

 one mucli less llocculent than the other ; the result was that the dura- 

 tion of sound in the first case was about a mmute, while in the second 

 it was ui^ward of two minutes. The darkness in the tunnel, on acc^iunt 

 of the smoke, was so profound immediately after the passage of a loco- 

 motive, that with two large torches, charged with mineral oil, the sides 

 of the tunnel at a distance of six feet could scarcely be observed ; while 

 in the other half of the tunnel, where no smoke existed, the eastern 

 opening could be observed like a star in the distance of upward of two 

 miles. It was therefore not surprising that the stifling of the sound 

 which was observed should be referred to the smoke as a palpable cause, 

 find that the more efl&cient one of the varying density or flocculent con- 

 dition should be disregarded. 



The method of determining by experiment the question as to which of 

 these causes was the efficient one did not occur to me until we had left 

 the tunnel, and then the simple expedient suggested itself to me, for the 

 purijose of repeating the experiment, that instead of locomotives charged 

 with wood, two locomotives charged with charcoal or coke — which emit 

 no smoke, but only transparent gases princii)ally carbonic acid — should 

 ho. used in an experiment similar to the one just described. This ex- 

 l)eriment Mr. Locke has kindly promised to perform as soon as a con- 

 venient opportunity shall occur. 



The opportunity was embraced while at the mouth of tlie tunnel to 

 make some observations which might have a bearing ux>on the phenom- 

 ena of the aerial echo. For this purj^ose, advantage was taken of a 

 large tool-chest, which happened to be place d about twenty or thirty 

 feet within the western mouth of the tunnel. By slamming down vio- 

 lently the cover of this chest, a lond sound of an explosive character 

 was produced, from which a prolonged echo was returned from the in- 

 terior of the tunnel. This echo was slightly intermittent, suddenly in- 

 creasing in loudness at intervals for a moment, and again resuming its 

 uniform intensity. This effect was attributed to projecting pieces of 

 rock in that x>artofthe tunnel which had not been lined with brick. 

 An echo was however evidently returned from that portion of which 

 the sides were not i^rojecting, which I wwdd consider an effect of the 

 same cause which produces the aerial echo. 



Aerial Echoes. 



During the year 1877, (as also in 187G,) series of exiDeriments were 

 made on the aerial echo, in which I was assisted — in the first series by 

 General Woodruff, engineer of tlie third light-house district, — and in the 

 second series by Edward Woodruff", assistant engineer of tlie same dis- 

 trict. These experiments were made principally at Block Island, but 

 also at Little Gull Island. Especial attention has been given to this 

 phenomenon, which consists in a distinct echo from the verge of the 

 horizon in the direction of the x>rolongation of the axis of the trumi^et 



