MECHANICS AND USEFUL ARTS. 59 



years may be obtained from tlie fact that the span of the sino^le 

 arch is oO feet wider than the span of the 2 arches which cover the 

 Great Northern Station. It has never before been attempted to 

 cover a space of 240 feet with 1 span for the purpose of forniin"' 

 a roof. In bridge-building much wider distances have been 

 spanned, but it has hitherto been thought unnecessary to covei'^o 

 wide a space with 1 roof. It is not very apj^arent why the at- 

 tempt has now been made, as 2 arches would have been much 

 more economical, and, we should have supposed, equally con- 

 venient. The height of the arch from the rail level is 99 feet, 

 and the rails 13 feet 6 inches above the road level. There will be 

 II lines of rails, and the space underneath is to be made available 

 for cellars, of which there will be about 4 acres. — English paper. 



FIRE-PROOF FLOORS. 



At different periods within the last few months a series of ex- 

 periments have been instituted, by Mr. Richard M. Hoe, for the 

 purpose of finding a cheap substitute for the iron beams and brick 

 now commonly used i\i fire-proof buildings. The last experi- 

 ment was made on Friday, May 15, at which many persons were 

 ^present, including members of numerous insurance companies 

 and officers of the fire department. The trial was made in a 

 vacant lot on Sheriff Street, New York, where a specimen floor 

 had been erected. This floor was 10 feet by 10, and was placed 

 upon upright posts of about 4 feet in height, beneath which, pre- 

 cisely at 9 o'clock, a large fire was kindled. Several men were 

 kept const;Tntly engaged in feeding the flames, and a high degree 

 of heat was attained. This was kept up to 12 o'clock, — 3 hours, 

 — when, there being no indications that the floor would be de- 

 stroyed, the fire was allowed to die out, and the men were per- 

 mitted to build one on the top. This being done, the flame was 

 rekindled below. These tAvo fires were now constantly supplied, 

 and the burning was continued until 4 o'clock, when all present 

 were satisfied of the successful issue of the experiment. The 

 floor was then taken to pieces and its construction explained. It 

 is about the usual thickness of floors separating different stories, 

 the lower surface being made of thin plain sheets of iron, No. 21 

 wire-gauge, which are nailed to the beams. Spread out upon 

 this sheet-iron ceiling w-as poured a layer of plaster of Paris to 

 the depth of 1^ inches ; on the top of the cross-beams were nailed 

 more sheets of iron, and upon these a layer of plaster of Paris 

 one-quarter of an inch in thickness. Above the whole were 

 fastened the planks, forming the upper surface of the floor. On 

 examination it was shown that the beams were not at all injured, 

 and this, too, after an intense flame had been burning steadily 

 above and below for more than 4 hours. The planks on the 

 upper part had, of course, been burnt, but that detracts but little 

 from the Avorth of tiie invention. The cost of a floor of the size 

 described above is $30.10; the cost of a building made in this 

 way will be about one-half as much as the brick and iron beam 



