1874] WRITINGS OP JOSEPH HENRY. 409 



this should he opened when the pump is not in use, in order to allow the 

 pipe to empty itself. 



When the draught-pipe cannot be protected from the cold, and the well 

 is at a considerable distance from the engine, the following expedient has 

 been employed with success : The pipe is enclosed in an India-rubber hose 

 of about double its diameter, and from time to time steam is forced through 

 the space between the hose and draught-pipe by means of a small pipe from 

 the boiler. 



Although the laws governing the reflection of light and heat arc undoubt- 

 edly in a great measure applicable to sound, there are yet so many disturb- 

 ing influences, such as inflection, refraction, (caused by the varying density 

 of the atmosphere,) &c., interfering with the reflection of the latter, that but 

 little use can be made of this property in directing and condensing the waves 

 of sound issuing from a fog-signal. This fact may bo illustrated by an ac- 

 count of some experiments made during the last year. 



A whistle being sounded in the focus of a large parabolic reflector, it was 

 very perceptible 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 whistle was increased, this disparity rapidly diminished, 

 and at the distance of a few hundred yards, entirely disappeared. The beam 

 of sound had been dissipated and the shadow had vanished. The effect of a 

 horizontal sounding-board 10 feet square, suspended over the whistle to pre- 

 vent the escape of sound in a vertical direction, was inappreciable at the 

 distance of a quarter 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 the frustum of a square 

 pyramid ; the larger base being 10 ft. by 10 ft., the smaller base 2 ft. by 2 ft., 

 and the length 20 ft. The axis was horizontal, and the whistle placed at the 

 smaller end. By this arrangement the increased power of the sound could 

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

 speaking-trumpet. 



It is probable that some modification of this form of whistle may be ad- 

 vantageously employed in certain localities, but there is however a disadvan- 

 tage attending the use of a trumpet 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 Daboll trumpet. The 

 sound from these signals being stronger on one course than any other, may 

 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 convinced that in some conditions of the atmosphere the most 

 powerful signals will be at times unreliable. 



Now it frequently occurs that a signal, which under ordinary circum- 

 stances would be audible at the distance of fifteen miles, cannot be heard 

 from a vessel at the distance of a single mile. This is probably due to the 

 reflection mentioned by Humboldt. 



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

 being very dilferent from that over the sea, the sound — in passing from the 

 former to the latter, undergoes reflection at their surfece of contact. The 

 correctness of this view is rendered more probable by the fact that when the 

 sound is thus impeded in the direction of the sea it has been observed to be 

 much stronger inland. 



When a vessel approaches a signal in a fog, a difficulty is sometimes ex- 

 perienced in determining the position of the signal bythe direction from 

 which the sound appears to proceed, the apparent and true direction being 



