APPENDIX. iii 



from Boston to St. John, the log-signals shall be sounded every 

 day on which the steamboats from these ports pass the station, 

 both in clear and foggy weather, the pilots on board these vessels 

 having, for a small gratuity, engaged to note the actual distance 

 of the boat when the sound is first heard on approaching the sig- 

 nal, and is last heard on receding from it. The boats above men- 

 tioned estimate their distance with considerable precision by the 

 number of revolutions of the paddle-wheel as recorded by 'the 

 indicator of the engine, and it is hoped by this means to definitely 

 decide the point in question. We think it highly probable that 

 fog does somewhat diminish the penetrating powei' of sound, or, 

 in other words, produce an efl'ect analogous to the propagation 

 of light. But when we consider the extreme minuteness of the 

 particles of water constituting the fog as compared with the 

 magnitude of the waves of sound, the analogy does not hold except 

 in so small a degree as to be of no practical importance, or, in 

 other woi'ds, the existence of fog is a true but, we think, an insuf- 

 cient cause of diminution of sound, which view is borne out by 

 the great distance at which our signals are heard during a dense 

 fog. 



Another cause, which without doubt is a true one, of the dimi- 

 nution of the penetrating power of sound is the varying density of 

 the atmosphere, from heat and moisture, in long distances. The 

 effect of this, however, would apparently be to slightly distort the 

 wave of sound rather than to obliterate it. However this may be, 

 we think, from all the observations we have made, the effect is 

 small in comparison with another cause, viz., that of the influence 

 of WIND. During a residence of several weeks at the sea-shore, the 

 variation in intensity of the sound of the breakers at a distance of 

 about a mile in no case appeared to be coincident with the varia- 

 tions of an aneroid barometer or a thermometer, but in every in- 

 stance it was affected by the direction of the wind. The variation 

 in the distinctness of the sound of a distant instrument as depending 

 on the direction of the wind is so marked that we are warranted 

 in considering it the principal cause of the inefficiency in certain 

 cases of the most powerful fog-signals. The elfect of the wind is 

 usually attributed, without due consideration, to the motion of 

 the body of air between the hearer and the sounding instrument : in 

 the case of its coming towards him it is supposed that the velocity 

 of the sound is reinforced by the motion of the air, and when in 

 the opposite direction that it is retarded in an equal degree. A 

 little reflection, however, will show that this cannot be the cause 

 of the phenomenon in question, since the velocity of sound is so 

 vastly greater than that of any ordinary wind, that the latter can 

 only impede the progress of the former by a very small percentage 

 of the whole. Professor Stokes, of Cambridge University, Eng- 

 land, has offered a very ingenious hypothetical explanation of 

 wind on sound, which we think has an important practical bear- 



(47) 



