Proceedings of the Polytechnic Association. 953 



for this reason in some cases the steam is condensedj and used oyer 

 and over again. 



Fuse for FiRmo Nitbo-Glycerine. 



Dr. D. D. Parmelee exhibited specimens of the fuse in general use 

 in exploding nitro-gl.yeerine. It is Mr. Bishops. The fuses are 

 primed with the mixture of the subphosphide and subsulphide of 

 copper with chlorate of potash, in which the terminals have not been 

 found quite free from adherent residue after ignition. The fuse is 

 ignited by a static electric machine which works in damp weather. 

 The disk of the machine is made of India rubber. Where a number 

 of explosions are required, the wires are alternately connected with 

 the positive and negative poles of the battery. 



The electrician of the Hoosic tunnel exploded several' primers, and 

 in one case he exploded four together^ the- report of which was simul- 

 taneous. 



An Old Photographic Camera. 



Mr. John Johnson exhibited a camera which was used some thirty 

 years ago. He also showed one made in 1S39, a year later, which, 

 was used in London .to take pictures. At first, a concave mirror 

 made of speculum metal, was- used ; this simply inverted the figure, 

 but did not change the right from the left. An objection to the 

 speculum was its high price, a good one costing $100 ; but the con- 

 cave mirror, here used, invented by Mr. Walcott, could be made for 

 about ten dollars. In 1810 he went to Londom and patented this 

 camera there, just one year before it was patented here. The first 

 cameras were of twelve inch focus, which was soon after reduced to 

 six inches, which made a picture in just half the time. With short 

 focus cameras, there is a difiiculty in getting the chemical focus ; a 

 tape was sometimes used to measure the distance from the sitter to 

 the camera, and then to place the index hand to that measure on the 

 scale attached to the camera. He only presented this camera as a 

 specimen of the first used in this country, and on account of its good 

 order and preservation. He first used the chloride of iodine on the 

 daguerreotype plate, and afterward the chloride of bromine, which 

 quickened the process materially. 



Bridges to Brooklyn. 

 Mr. Fisher made a diagram on the black-board, of the suspension 

 bridge, to start from Tryon Row, the ground here being thirty feet 

 above high water in the middle of the river. 



