1874] WRITINGS OF JOSEPH HENRY. 381 



less perceptible, and at the limit of audibility, the addition 

 of the reflector appeared to us entirely imperceptible. This 

 result was corroborated by subsequent experiments in which 

 a whistle was heard nearl}^ as well in the rear of a reflector 

 as before it. It would appear from these results that while 

 feeble sounds at small distances are reflected as rays of light 

 are, waves of powerful sound spread laterally, and even 

 when projected from the mouth of a trumpet tend at a great 

 distance to embrace the whole circle of the horizon. 



Upon this and all the subsequent experiments, as it will 

 appear, the principle of reflection as a means of re-inforcing 

 sound is but slightly applicable to fog-signals. It is evident 

 however that the efiect will be somewhat increased by 

 augmenting the size of the reflector and by more completely 

 inclosing the source of sound in a conical or pyramidal 

 reflector. 



Another series of experiments was made to ascertain 

 whether the penetration of the sound was greater in the 

 direction of the axis of the bells, or at right angles to the 

 axis; or in other words, whether the sound was louder in 

 front of the mouth of a bell or of its rim. The result of this 

 experiment was considered of importance, since in one of the 

 light-houses a bell has been placed with the plane of its 

 mouth at right angles to the horizon, instead of being placed 

 as usual parallel to the same. The effect on the sound in 

 these two positions was similar to that produced by the bell 

 with a reflector, the noise at a short distance being greater 

 with the mouth toward the observer than when the rim was 

 in the plane of the ear. At a considerable distance however, 

 the difference between the two sounds was imperceptible. In 

 practice therefore it is of very little importance whether the 

 axis of the bell is perpendicular or parallel to the horizon. 



The first fog-signal examined in this series of experiments 

 was a double whistle, improperly called a steam-gong, de- 

 signed principally for a fire-alarm and for signals for the 

 commencement of working-hours in large manufacturing 

 establishments. It consisted of two bells of the ordinary 

 steam-whistle on the same hollow axis, mouth to mouth, 



