MECHANICS AND -USEFUL ARTS. 87 



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sure, the change in weight by absorption in three years being 

 found to be only 1 per cent, in thin sheets. The material is first 

 formed into massive cylinders, then shaved from 'the periphery by 

 keen cutting machines into very thin, long sheets, next slit into 

 tapes, and these are served around the conductor to the thickness 

 desired, when the mass is welded by heating. The secret of the 

 improvement, however, is said to be in the perfect elimination of 

 oxygen, imperfectly effected by vulcanization, which was the 

 element that caused the gradual softening and permeability hith- 

 erto experienced in the article. It was a process of slow oxida- 

 tion or decay, which has been entirely obviated. The removal 

 of oxygen leaves the caoutchouc of its proper color, pure white. 

 Scientific American. 



THE MAGNESIUM LIGHT. 



Magnesium has recently been applied with the greatest success 

 in pyrotechny. The powdered metal, when covered with par- 

 affine, is preserved from the action of acids and alkalies, and can 

 safely be employed in the manufacture of fireworks. If only from 

 2 to 5 per cent, of magnesium be mixed with the ordinary rocket- 

 powder the light is greatly intensified ; and the effect was seen to 

 great advantage in recent pyrotechnical displays at the Crystal 

 Palace. In rockets the dense white smoke produced by magne- 

 sium is an advantage rather than a loss ; for the canopy then seen 

 floating like a network of snow-white gauze over each burning 

 star not only adds by its appearance to the beauty of the display, 

 but reflects downward an additional amount of light. The use of 

 the magnesium powder in rockets for signals at sea deserves the 

 investigation of the authorities at Woolwich, as the light is so 

 greatly intensified at so small a cost. The American government 

 is now seriously considering the desirability of adopting the mag- 

 nesium light as their signal light for the service, both in lamps 

 and out of them. At the Wimbledon meetings the light was em- 

 ployed with good effect, the magnesium balloons being generally 

 mistaken by the public for meteors. British Journal of Photog- 

 raphy. 



PROSSER'S LIME LIGHT. 



The lime light has recently been introduced, on trial, into light- 

 houses, where it promises to prove a formidable rival to Holmes' 

 electric light. At the South Foreland Point, Mr. Prosser's lamp 

 for the production of lime light was placed three years ago in the 

 upper lighthouse. The lamp was fixed in the centre of the Fres- 

 nel apparatus, which had already been employed with the electric 

 light, and which was adjusted to the use of a four- wicked oil lamp, 

 the burner of which was 3| inches in diameter. We take from 

 the report of Professor Faraday, made to the Trinity House, on 

 the llth of June, last year, an account of the light, in regard to 

 its working and success in the Upper Lighthouse at the South 

 Foreland : 



" The lamp consists of a central octahedral prism of quicklime, 



