C. GENERAL PHYSICS. 129 



being kept in place between slides of microscope glass. 

 The effect of the heat from the electric discharge is to vapor- 

 ize the metal, which is instantly condensed in a transparent 

 layer upon the cold glass, which can then be studied by the 

 microscope, and can be used in various ways to determine 

 the character of the metal and the peculiarities of the dis- 

 charge. 12 A, X., 190. 



STEAM FOG-WHISTLES. 



It has been found by General Duane, of the United States 

 Engineers, in his experiments made to determine the best 

 form of boilers for steam fog-signals, that as the steam used 

 is at a high pressure, and is drawn off at intervals, there is 

 a constant tendency to foam and throw out water with the 

 steam. To counteract this, a horizontal tubular boiler, like 

 those used in locomotives, is recommended by him. The 

 steam-dome must be very large, and surmounted by a steam- 

 pipe 12 inches in diameter. The steam should be drawn 

 off at a point ten feet above the water level in the boiler. 

 The diameter of the boiler whistle should be two thirds of 

 its length, and the vertical distance of its lower edge above 

 the coping, for a steam pressure of 50 pounds, should be 

 from one third to one fourth of the diameter. Elliofs Eu- 

 ropean Lig J it-house System, p. 25. 



THE GAS GUN FOR FOG-SIGNALS. 



A very ingenious application has been made, by Mr. 

 "VVigham, of the explosive nature of a mixture of ordinary 

 gas in air. He establishes at any point on the coast where 

 a fog-signal is desired a gas gun. It is simply a tube of 

 iron, connected with the gas-holder by the proper pipe ; the 

 holder, of course, may be at any convenient distance. The 

 gas-holder is filled with a mixture of one fourth air, and the 

 remainder coal-gas and oxygen, and this mixture is allowed 

 to flow into the gas gun, when it may be fired off by touch- 

 ing a match to the proper orifice, taking care, of course, to 

 close all communication with the holder. By using an elec- 

 tric spark, instead of the match, the service of the gun may 

 be made still easier. The flash from this gun is said to 

 illuminate the fog much better than that from a discharge 

 of gunpowder. Elliots European Light-house System, p. 74. 



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