Ordinary Window-Glass.

Window-glass is sold by the box, which contains, as nearly as may be, fifty square feet, whatever may be the size of the panes. The thickness of ordinary, or "single thick," window-glass, is about one-sixteenth of an inch, and, of "double thick," nearly one-eighth of an inch.

The tensile strength of common glass varies from 2000 pounds to 3000 pounds per square inch, and its crushing strength from 6000 pounds to 10,000 pounds.

The following table gives the number of panes of window-glass in one box, or fifty feet:

Where skylights are glazed with clear or double thick glass, it y be used in lengths of from sixteen to thirty inches by a width rom nine to fifteen inches. A lap of at least an inch and a half

is necessary for all joints. This is the cheapest mode of glazing. The best, however, for skylight purposes, is fluted or rough plateglass. The following thicknesses are recommended as proportionate to sizes:

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12 inches by 48 inches is the extent for glass inch thickness.

Asphaltum is used extensively for composition roofing, for the same purpose as tar.

Asphaltum, or solid bitumen, is a natural pitch, found in different countries. The most accessible and economical for use in the United States is obtained from the "Great Pitch Lake," a remarkable and inexhaustible deposit in the island of Trinidad.

It is impervious to water, and is one of the most unchangeable and durable substances known, qualities which, together with its tenacity, adhesiveness, and resistance to the effects of the most extreme changes of heat and cold, make it a cementing material of the greatest value for roofs, pavements, and various other purposes. The principal advantages claimed for asphaltum as a roofing material over pitch and coal-tar, arise from the fact that the bituminous matter of the asphalt is not volatile at any temperature of the sun's heat, and is therefore permanent; while in all materials manufactured from coal-tar there are volatile oils, which slowly evaporate on exposure to the sun and air, destroying the flexibility and life of the material. The fact is now well known, that any pitch or cement manufactured from coal-tar thus gradually deteriorates, until, in the course of years, it becomes brittle, and crumbles away; and that felt saturated with coal-tar in like manner hardens, until it becomes brittle and finally worthless.

Asphalted sheathing-felt, for roofing purposes, and for laying under shingles, slates, clapboards, etc., is also made in a similar manner to the tarred papers more commonly used for the above purposes. Both these materials may be found in the mar ket, in a condition ready for use.

ROCK ASPHALT.

Rock asphalt is a lime ore impregnated naturally, by a geological phenomenon still but imperfectly explained, with bitumen in the proportion of 6 to 10 for 100. It is found in strata like coal. It exists in Europe in many places, and is a material relatively rare, but of great value. It is mined principally at Seyssel and Pyrimont, in the valley of the Rhone, France, and in the Val de Travers, canton of Neuchâtel, Switzerland, and at Ragusa, on the island of Sicily.

If a piece of asphaltic ore be exposed to a temperature of from 80° to 160° centigrade, it will become powder. The bitumen, which serves to keep together the molecules of lime, softened by the heat, begins to melt; and, their cohesion thus destroyed, the grains of lime, each coated with a pellicle of bitumen, separate, and form a chocolate-colored powder. If, while it is yet hot, this powder is put into a mould, it will re-assume, as soon as it is cold, its former consistency; and the block of ore will have been reconstructed with its same grains, and, in general, its same properties. It is upon this singular property that the principle of laying roadways of compressed asphalt is founded.

If, instead of treating asphalt as explained above, it should after being broken be heated in kettles (in which a little bitumen has been first put to serve as a foundation) for five or six hours, there will be obtained a sort of black semi-liquid paste, which is mastic asphalt. This is the material which, being mixed with a little gravel, is used for laying walks, floors, roofs, etc. In this operation the bitumen which is first put in the kettle plays the same part as grease in a frying-pan. It stops the asphalt from burning before it is melted, while at the same time it restores the bitumen that the asphalt has lost by evaporation.

The paste referred to is then put in the moulds, varying in shape according to the use for which the asphalt is prepared. Generally each of the cakes thus formed bears the manufacturer's mark, which is of great use in detecting frauds, for nothing is easier to imitate than real mastic asphalt. With a little bitumen and some macadam powder, any one can make a block that the most practised eye cannot tell from the genuine. It is time alone which denounces the imposture, and often at a disastrous price to those so delu led. Compressed asphalt has been long used in Europe for carriageways, sidewalks, and courtyards, subject to considerable traffic.

Mr. E. P. North, C.E., member of A.S.C.E., in his report on the avements of London and Paris (“Transactions American Society Civil Engineers," clxxx. p. 126), says "From a sanitary point

of view, asphalt is without a peer. Its surface is smooth, regular, and non-absorbent, with no cavities or cracks of any kind to retain the infected mud and dust of the streets, and the soil beneath it is kept dry. It is more thoroughly cleaned, either by sweeping or washing, than any other pavement. Its freedom from noise, and its other excellences, are fast placing it in all the business and banking streets of the city of London, where it seems to be superseding all other pavements. In comparison with granite, its great economy is to brain-workers and the owners of horses." In an article in Johnson's Cyclopædia, Gen. Q. A. Gillmore says of the "natural rock asphalt,"

"It must be conceded that nothing has yet been discovered which can replace with entire satisfaction the bituminous limestones of Seyssel and Val de Travers and Sicily. In the natural asphaltic rock, the calcareous matter is so intimately and impalpably combined with the bitumen, resists so thoroughly the action of air and water and even muriatic acid, is so entirely free from moisture, properties due, perhaps, to the vast pressure and intense heat under which the ingredients have been incorporated by nature, that we are forced to attribute the excellence of this material to the existence of certain natural conditions which the most skilful artificial methods fail to reproduce."

Mastic asphalt is used for floors of cellars, stores, breweries, malt-houses, hotel kitchens, stables, laundries, conservatories, public buildings, carriage-factories, sugar-refineries, mills, rinks, etc.; and for any place where a hard, smooth, clean, dry, fire an 1 water proof, odorless, and durable covering of a light color is required, either in basement or upper stories. It can be laid either over cement concrete, brick, or wood, in one sheet without seams: also over cement concrete for roofs, for fire-proof buildings. For dwelling-house cellars, especially on moist or filled land, this material is especially adapted, being water-tight, non-absorbent, free from mould or dust, impervious to sewer-gases, and for sanitary purposes invaluable.

Mastic asphalt is also valuable for damp courses over foundations, and for covering vaults and arches underground.

The use of asphalt for roofs is extending, many of the principal buildings in London and a large number in this country being covered with it. It possesses especial advantages for this purpose from the fact that it is both fireproof and fire-resisting.

Architects and builders desiring to employ asphalt for any of the above purposes should be careful to secure the genuine Val-deTravers or Seyssel or Sicilian rock asphalt, as there are imitations which are of but little value.

For floors of cellars, courtyards, etc., laid on the ground, a base of cement concrete 3 inches thick should first be laid; and over this is put a layer of asphalt from to 14 inch thick, according to the use to which it is to be put. For ordinary cellar floors, the asphalt need not be more than inch thick: for yards on which heavy teams are to drive, it should be 14 inches thick. In specifying asphalt pavement, both the thickness of the concrete and of the asphalt should be given: it should also be remembered, “ asphalt pavement does not include the concrete foundation unless so specified.

In laying asphalt over plank or boards, a layer of stout, dry (not tarred) sheathing-paper should first be put down, and the asphalt laid on this. Asphalt floors for stables should be at least 1 inch thick. The cost of rock asphalt in the large cities varies from 15 to 20 cents per square foot in jobs of 2,000 feet and over. This does not include the concrete foundation. Imitation asphalts are laid for considerably less, and German and other cheap asphalts for about two-thirds the above price.