Alas, the “temporary” addition to that old Dutch church on Nassau street has but too truly proved a prediction, and we of the Empire city will either have to put up with the present arrangement, or build a postal structure of our own. It is evident now that the act of Congress, in this case made and provided, is but A DEAD LETTER. SARAH B.—In the case you mention, the lightning rod was secured to the wall of the house by iron staples. There was nothing to hinder the electric fluid from turning off on one of these, as it actually did. Accidents of a like nature are constantly happening, and where sufficient precaution is not taken it would be far safer to have no conductor. S. T., asks, is there any bank lock, of how many, and whatever combinations, that is absolutely secure against thoroughly posted and prepared burglars? We doubt that there is. For years the Bank of England trusted its vaults, filled with treasures, to the celebrated _Chubb_ lock. Yet that ingenious Yankee, Hobbs, opened it in a surprising short time. The fact is—what man’s ingenuity can make, man’s ingenuity can also unmake. C. G., Cincinnati.—We perfectly agree with you; the dwellings of this day are really combustible, and highly dangerous; much more so in fact than before burning fluids came so much into use. We also agree with you that the roofs of houses should not be of a material so liable to take fire on the occasion of a pyrotechnical display, or the passing of a spark-emitting locomotive. Shingles could be easily rendered fire-proof by steeping them, before use, in a strong solution of alum. But most people would willingly “lose the sheep, to save the pennyworth of tar.” R. D., Baltimore.—The silica coating of any building material renders it very durable. It is the combination of carbonate of lime, or chalk, with silicate of soda, or what is more commonly known as “soluble glass,” and by the old chemists called “oil of flint,” which, under heavy pressure, produces extraordinary hardness, and causes the great adherence of this cement to iron, brick, stone, or wood. And it is but one more proof of the practical property of the silicate, when applied to purposes such as those in which building most requires its valuable aid.