Scientific Lectures. 235 



prevails ; care is not taken to lap the joints. Smoke flues should 

 always be covered by two thicknesses of brick, and both the verti- 

 cal and horizontal joints of the inside tier should be covered by the 

 outer tier, so that no flame and no sparks can possibly escape through 

 spaces left unfilled by mortar. 



As a substitute for two thicknesses of brick, and one which offers, 

 I think, greater security, the smoke flues may be formed of cement 

 pipes, well jointed with a single layer of bricks over them. 



Fire-proof Buildings. 

 Our system for constructing fire-proof buildings is very good, 

 though no building can be called fire-proof which is not protected by 

 well made iron shutters. Our rolled iron beams, twelve or fourteen 

 inches high, and brick arches, make fire-proof buildings too expensive 

 for many purposes; banks, railroad corporations, insurance com- 

 panies and rich firms can afford them, but hotels can rarely afford 

 them, and they are out of the question for private dwellings. The 

 increased expense is not due to the iron beams and arches alone, 

 but to the fact that the enormous weight of the floors makes it neces- 

 sary to build deeper and more massive foundations, and thicker 

 walls. Nothing is more desirable than a system of cheap semi-fire- 

 proof buildings ; buildings which, while not sufficiently fire-proof to 

 withstand a great conflagration, should be so non-inflammable as to 

 make great conflagrations impossible. A great fire like that which 

 destroyed so much of Chicago would be impossible in most European 

 cities, simply because the buildings are so non-inflammable that fire 

 rarely passes from one to another. Almost every house in Paris is 

 semi-fire-proof. Even with the aid of petroleum, the communists 

 failed to destroy any large portions of the city. Stone, iron and 

 plaster of Paris are the building materials chiefly employed there, 

 with some bricks and but little wood, the latter hard wood. The 

 floors are made of iron and plaster of Paris, with a covering of 

 boards above. Light iron beams, not more than three or four inches 

 high, are set about three feet apart ; at right angles to .these, about 

 three feet apart, run short, light flat iron bars which hook at the ends 

 over the beams, and hang even with their under edges. Parallel 

 with the beams, and loosely resting on the cross bars, run strips of 

 hoop iron, two or three between every two beams. "When this light 

 iron net work is complete, a temporary table is placed in contact with 

 the under side and plaster of Paris poured in from above, nearly even 

 with the upper edges of the beams; this sets hard in a few minutes, 



