APPENDIX. VH 



had been blowing violently towards Albany for twenty hours, it 

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

 it met the icest 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 mentioned by General Duane, and 

 which, from the frequency with which he has witnessed the occur- 

 rence, we must accept as a fact, tlwugh 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 sur- 

 face 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 audible 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 abnormal propagation of 

 sound. 



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

 fog-signal, bells, gongs, etc., are too feeble except in special cases 

 where the warning required is to be heard only at a small dis- 

 tance. 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 depend 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 produces an effect 

 identical with that from one of metal. Another variety is the 

 fog trumpet, which consists of a trumpet of wood or metal actu- 

 ated by a reed like that of a flageolet. The 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 number of holes 

 through it. This drum is placed at the mouth of a large trum- 

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

 the air by the opening and shutting of the orifices and consequent 

 rushing out at intervals with explosive violence of the steam, or 

 condensed air. The instrument, as originally invented by Craig- 

 nard De la Tour of France, was used simply in experiments in 

 physics to determine the pitch of sound ; but Mr. Brown of New 

 York, after adding a trumpet to it, and modifying the openings 



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