180 ANNUAL RECORD OF SCIENCE AND INDUSTRY. 



hydrogen and oxygen, which determines the vibration of the 

 air in the tube. In order that the sound be produced in all 

 its intensity, it is necessary and sufficient that the number 

 of detonations produced by the molecules of oxygen and hy- 

 drogen in a given time shall be in accord with the number 

 of vibrations corresponding to the sound produced by the 

 tube. Pie finds it sufficient then to increase the number of 

 his flames, substituting four, five, six, or more jets of illumi- 

 nating gas for his two jets of hydrogen, and diminishing the 

 height of these flames correspondingly, until the sum total 

 of the surfaces of the photospheres suffices to produce the 

 vibrations of the air in the tube. Bull. Ilebd., 1875, 266. 



RELATIVE EFFICIENCY OF VARIOUS FOG-SIGNALS. 



The principal instruments employed on the American coast 

 as fog-signals are the Daboll reed trumpet, the locomotive 

 whistle, and the siren. In a report on the relative efficiency 

 of these instruments, General Duane states in reference to 

 all of them that, while they are frequently heard at distances 

 of twenty miles, yet as frequently they can not be heard a 

 distance of two miles, and this with no perceptible difference 

 in the state of the atmosphere. It is therefore very difficult 

 to determine the relative powers of fog-signals, unless they 

 are placed side by side, under exceptionally favorable at- 

 mospheric circumstances. The sound from the whistle is 

 equally distributed in all horizontal directions, and is most 

 powerful in a horizontal plane passing through the whistle. 

 The sound from the siren is most distinct in the axis of the 

 trumpet with which it is provided. The sound given by the 

 Daboll reed trumpet is usually strongest in a plane perpen- 

 dicular to its axis. In the average of a great number of ex- 

 periments, General Duane concludes that the powers of the 

 first-class siren, the 12-inch whistle, and the first-class Daboll 

 trumpet may be expressed by the numbers 9, 7, and 4. The 

 extreme limit of the audibility of the sound of the trumpet 

 is twelve miles; that of the 12-inch whistle about twenty 

 miles. That of the siren has not been ascertained. The rel- 

 ative expenditure of fuel by the steam-engines working these 

 instruments at their full capacity is, for the siren, 9 ; the 

 whistle, 3 ; and the trumpet,!. As regards the skill and 

 attention required in the management of these signals, the 



