L. MECHANICS AND ENGINEERING. 467 



inch lower still. He is not able to establish a definite period 

 in the flow, although its irregularity is abundantly evident. 



FOG-SIGNALS. 



Professor Joseph Henry, chairman of a committee on fog- 

 siomals, has communicated a number of interesting: observa- 

 tions made by him on the phenomena of sound as relating to 

 the subject in hand. In studying the subject of fog-signals, 

 it becomes a question of importance to ascertain whether 

 waves of sound, like those of light, are absorbed or stifled by 

 fog. On this point observers disagree; and to settle this 

 question definitely the assistance of the pilots of the boats 

 running between Boston and St. John, New Brunswick, has 

 been secured, and they have promised to note the actual dis- 

 tance of a body from a given fog-signal when the sound is 

 first heard on approaching, and again when it is lost on re- 

 ceding from it. Professor Henry considers it highly proba- 

 ble that fog does somewhat diminish the penetrating power 

 of sound, but only to an exceedingly minute degree. Among 

 the principal causes of the diminution in this penetrating 

 power are enumerated the varying density of the atmosphere, 

 the direction of the wind, and the reflection of the sound 

 wave from neighboring objects, such as hill -sides, forests, 

 houses, etc. According to General Duane, it appears that 

 although a reflector, in the focus of which a steam-whistle or 

 ordinary bell is placed, reflects the sound a short distance, it 

 produces little or no effect at the distance of two or three 

 miles. In the case of signals that were sounded at the side 

 of a bank with a large house directly in the rear, the roof of 

 which would tend to deflect the sound forward, it was shown 

 that this sound shadow vanishes at the distance of a mile and 

 a half or two miles, and that at the distance of three miles 

 the sound was quite loud. The fog-signals have frequently 

 been heard at the distance of twenty miles, and as frequently 

 can not be heard at the distance of two miles, and this with 

 no perceptible difference in the state of the atmosphere. This 

 case, although quite abnormal, seems to be sufficiently well 

 authenticated to justify its publication under the authority 

 of the Light-house Board. The instruments employed as fog- 

 signals by this board are mainly thi*ee, all constructed on a 

 principle of resounding cavities, in which the air is the sound- 



