4X0 WRITINGS OF JOSEPH HENRY. [1874 



entirely different. This is undoubtedly due to the refraction of sound pass- 

 ing through media of different density. 



Experiments and observation lead to the conclusion that these anomalies 

 in the penetration 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 in- 

 fluence than has generally been supposed. 



In the foregoing I differ entirely in opinion from General 

 Duane as to the cause of extinction of powerful sounds being 

 due to the unequal density of the atmosphere. The velocity 

 of sound is not at all affected by barometric pressure, 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 I think is entirely too minute to produce 

 the results noted by General Duane, 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., 2 J inches, 

 3 ins., 4 ins., 5 ins., 6 ins., 10 ins., 12 ins., and 18 ins. in diameter. Those of 

 2J ins., 3 ins., 5 ins., and 10 ins. were fitted (instead of the ordinary bell) with 

 long cylinders provided with movable pistons, so that the effective length of 

 the bell could be altered at pleasure. The pitch of the blast was found to vary 

 with the length of the bell, and the power of the whistle with its diameter. 

 The ratio of the power to the diameter was not accurately obtained, but it 

 is probable that the extreme range of sound of a whistle is proportional to 

 the square root of its diameter. 



This result (that the pitch varies with the length of the 

 bell,) is in conformity with well-established principles of 

 resounding cavities; and that the power should increase 

 with the extent of the aerial reed (the vibrations of which 

 give motion to the resounding air within the cavity,) is also 

 as we have seen, in accordance with hypothetical considera- 

 tions : but as the density of this stream of steam (and con- 

 sequently the rapidity of its vibrations) depends upon the 

 pressure of the steam in the boiler, a perfect whistle should 

 have the capability of changing its dimensions, not only in 

 relation to the width of its throat, but also in regard to the 

 pressure of the steam in the reservoir. 



The pitch giving the greatest range appears to be at the middle of the 

 scale of sound. It is certain that a good result cannot be obtained from 

 either a very shrill or a bass note. This remark is applicable to all varieties 

 of signal. 



