RESEARCHES IN SOUND. 495 



vationof a iiiileor two. This is shown by the rai^id eastwarclly motion 

 of the upper clouds as occasionally seen through breaks in the lower. 



As a further illustration of this principle I may mention that on one 

 occasion (in 1855) I started, on my way to Boston from Albany, in the morn- 

 ing of a clear day, with a westerly wind. The weather continued clear 

 and pleasant until after passingthe Connecticut Eiver, and until within 

 fifty miles of Boston. We then encountered a storm of wind and rain 

 which continued until we reached the city. On inquiry I learned that 

 tlie storm had commenced in Boston the evening before, and, although 

 the wind had been blowing violentlj^ towards Albany for itcenty hours, 

 it had not reached inwardly more than fifty miles. At this point it met 

 the west wind and was turned back above in almost a parallel current. 

 This is the general character of northeast storms along our coast, as 

 shown by Mr. Espy, and is directly applicable to the phenomenon men- 

 tioned by General Duane, and which, from the frequency with which he 

 has witnessed the occui-rence, we must accept as a fact, though by no 

 means a general one applicable to all stations. While a violent wind was 

 blowing towards his place of observation from Cape Elizabeth, at the 

 surface of the earth, a parallel current of air was flowing above with 

 equal or greater velocity in the opposite direction. The effect of the 

 latter would be to increase the velocity of the upper part of the wave 

 of sound, and of the former to diminish it ; the result of the two being 

 to incline the front of the wave of sound towards the observer, or to 

 throw it down towards the earth, thus rendering the distant signal audi- 

 ble under these conditions when otherwise it could not be heard. I think 

 it is probable that the same principle applies in other cases to the abnor- 

 mal propagation of sound. 



For the production of a sound of sufficient power to serve as a fog- 

 signal, bells, gongs, &c., are too feeble except in special cases where the 

 warning required is to be heard only at a small distance. After much 

 experience, the Light-House Board has adopted, for first-class signals, 

 instruments actuated by steam or hot-air engines, and such only as de- 

 pend upon the principle of resonance, or the enforcement of sound by a 

 series of recurring echoes in resounding cavities. 



Of these there are three varieties. First, the steam-whistle, of which 

 the part called the bell is a resounding cavity, the sound it emits having 

 no relation to the material of which it is composed ; one of the same 

 form and of equal size of wood producing an effect identical with that 

 from one of metal. Another variety is the fog-trumpet, which consists 

 of a trumpet of wood or metal actuated by a reed like that of a clario- 

 net. Tlie third variety is called the siren trumpet, which consists of a 

 hollow drum, into one head of which is inserted a pipe from a steam- 

 boiler, while in the other head a number of holes are pierced, which are 

 alternately opened and shut by a revolving plate having an equal num- 

 ber of holes through it. This drum is placed at the mouth of a large 

 trumpet. The sound is produced by the series of impulses given to the 



