MECHANICS AND USEFUL ARTS. 103 



cotton, 8 grains; nitrate of silver, 30 grains; dissolved in as 

 small a quantity of water as possible, and allowed to settle for a 

 few days, protected from the light. Flow the salted block with 

 formula No. 2, in the dark room, and dry it by a gentle heat. It is 

 now ready for exposure under the negative. A porcelain print- 

 ing-frame, or any other suitable method, may be used to print it. 

 After printing, formula No. 2 is removed from the surface of the 

 block by dissolving it in ether and alcohol, assisted by rubbing 

 gently with a soft sponge. The picture can now be toned and 

 fixed in the ordinary way, or fixed and toned at one operation, 

 by the hypo and gold bath. After being allowed to dry, it is 

 ready for the engraver. 







THE OXYGEN LIGHT. 



According to the " Opinion Nationale," Paris, the new Prefet 

 de la Seine has definitively authorized the Tessie du Motay Com- 

 pany to lay their underground communications in the city of 

 Paris for illuminating with oxygen gas. 



A system of pipes will connect the oxygen works of Pantin 

 with the boulevards, and in a few months all the inhabitants re- 

 siding between the "New Opera" and the Passage Jouffroy will 

 thus be enabled to benefit from the immense advantages ottered 

 by this new light over the old gas. 



Already oxyhydric lanterns have been placed at the entrance of 

 the bazaar European, near the Passage Jouffroy, and project a 

 light of the purest white and the most dazzling brilliancy, near 

 which the old gas pales and appears to shine with the most singu- 

 lar yellow color. 



The journal referred to congratulates M. le Prefet de la Seine 

 for having ratified a measure in accordance with the general 

 wishes and interests of the people, and which appears to it to be 

 the indispensable corollary of the great improvements under- 

 taken within a few years in Paris. 



USE OF CALCIUM LIGHTS AT THE ST. LOUIS BRIDGE. 



Mr. W. Milnor Roberts, who is in charge of the work, says : 

 " We have used calcium lights only for our open-air work in lay- 

 ing masonry on the top of our caissons, one light on one side, 

 and one at the other, on diagonal corners ; we found that thev 



c * / 



distributed the best light when thus placed. We had the oxygen 

 gas forced into copper gas-holders with a pressure of about 200 

 pounds to the square inch. These were carried over from the 

 city to the piers on a little steamer, and the gas was conveyed to 

 the burner through small lead pipe. At first our reflectors were 

 of glass, but so many were broken that they were replaced by 

 metal. A man remained with the two burners through the night, 

 to regulate them occasionally, and to mend the pipes when a 

 burst occurred. They usually burn from 11 to 12 hours; and, 

 with the aid of some movable large reflector lamps, the masons 



