EESEAECHES IN SOUND. 485 



direoting and condensing the waves of sound issning from a fog-signal. This fact may 

 he ilhiatrated hy an account of some experiments made during the Last year. 



A whistle heiug sounded in the focus of a large parabolic reflector, it was very per- 

 ceptible to an observer in the immediate vicinity that the sound was louder in the 

 front than in the rear of the reflector. As the distance of the observer from the whistlo 

 was increased this disparity rapidly diminished, and at the distance of a few hunch'ed 

 yards entirely disappeared. The ?)Cff?n of sound had been dissipated and the shadow 

 had vanished. The eftect of a horizontal sounding-board 10 feet square, suspended over 

 the whistle to prevent the escajjo of sound in a vertical direction, was inappreciable at 

 the distance of a qtiarter of a mile. 



The employment of a trumpet with the whistle was rather more successful. The 

 trumpet was constructed of wood, in the form of a square pyramid ; the lower base 

 being 10' Ity 10', the upper base 2' by 2', and the height 20'. The axis was horizontal 

 and the whistle placed at the smaller end. By this arrangement the increased ]iower 

 of the sound could be perceived at the distance of a mile, the action being similar to 

 that of a speaking-trumpet. 



It is prol)able that some modification of this form of whistle may be advantageously 

 employed in certain localities, but there is however a disadvantage attending the use 

 of n Irumpet with fog-signals. 



The sound from a trumpet not being uniformly distributed, it is difficult to estimate 

 the distance of the signal, or,.as the pilots term it, " to locate the sound." This has 

 been observed in the siren and DaboU trumpet. The sound from these signals being 

 stronger on one course than any other, maj^ be distinctly heard from a vessel when 

 crossing the axis of the beam of sound, but as its distance from this line increases, the 

 sound appears fainter and more remote, although the vessel may be approaching the 

 signal. 



From an attentive observation, during three years, of the fog-signals on this coast, 

 and from the reports received from captains and pilots of coasting vessels, I am con- 

 vinced that in some conditions of the atmosi>here the most powerful signals will be 

 at times unrehable. 



Now it frequently occurs that a signal, which under ordinary circumstances would 

 be audible at the distance of fifteen miles, cannot be heard from a vessel at the dis- 

 tance of a single mile. This is probably due to the reflection mentioned by Hum- 

 boldt. 



The temperature of the air over the land where the fog-signal is located, being very 

 different from that over the sea, the sound, in i>assing from the former to the latter, 

 undergoes reflection at their surface of contact. The correctness of this view is ren- 

 dered more j)robable by the fact that when the sound is thus imx)eded in the direction 

 of the sea, it has been observed to bo much stronger inland. 



When a vessel approaches a signal in a fog, a difficulty is sometimes experienced in 

 determining the position of the signal by the direction from which the sound appears 

 to proceed, the apparent andtxue direction being entirely different. This is undoubt- 

 edly due to the refraction pf sound passing through media of different density. 



Experiments and observation lead to the conclusion that these anomalies in the pen- 

 etration and direction of sound from fog-signals are to be attributed mainly to the want 

 of uniformity in the surrounding atmosphere, and that snow, rain, fog, and the force and 

 direction of the wind, have much less influence than has generally been supposed. 



[In tlie foregoing I dififer entirely in opinion from General Duaue as 

 to the cause of extinction of i^owerful sounds being due to the unequal 

 density of the atmosphere. The velocity of sound is not at all affected 

 by barometric i^ressure, but if the difference in pressure is caused by a 

 difference in heat, or by the expansive power of vapor mingled with the 

 air, a slight degree of obstruction of sounds may be observed. But this 

 effect we think is entirely too minute to produce the results noted by 

 General Duaue, while we shall find in the action of the currents of wind 

 above and below, a true and sufficient cause.] 



The experimental whistles were of the following dimensions, viz: 2V, 3", 4", 5", G", 

 10", 12", and 18" iu diameter. Those of 2^", 3'', 5", and 10" were fitted, instead 

 of the ordinary bell, Avitli long cylinders, i^rovided with movable pistons, so that 

 tlie effective length of the bell could bo altered at pleasure. The pitch of the 

 blast was found to v.ary with the length of the bell, and the power of the whistle 

 witli its diameter. The ratio of the power to the diameter was not accurately ob- 

 tained, but it is proliablc that the extreme range of sound of a whistlo is proportional 

 to the square root of its diameter. 



