A barrel of Portland cement weighs about 375 lb. net; one of the Eastern Rosendales, 300 lb., and Western Rosendales (from Wisconsin, Kentucky, Illinois, etc.), about 265 lb. A cubic foot of slightly compacted cement mixed with cu. ft. of water will make from to cu. ft. of paste; or 1 bbl. of cement will make about 3 cu. ft. of stiff paste.

CONCRETE.

A very good concrete may be made by using the following quantities of materials, which, when mixed, will make 1 cu. yd.: 2 bbl. of Rosendale cement, cu. yd. of sand, and cu. yd. of broken stone.

The strength of concrete increases considerably with age. For example, a Portland-cement concrete, 1 month old, will crush under a load of about 15 tons per sq. ft., while if it is a year old, it will sustain about 100 tons per sq. ft. Natural. cement concretes, of ages from 6 months to 4 years, showed crushing strengths of from 70 to 100 tons per sq. ft.

THICKNESS OF WALLS.

The thickness of foundation walls in all the large cities is controlled by the building laws. Where there are no exist ing laws, the following table will serve as a guide:

TIN ROOFING.

There are two kinds of tin-roof coverings in common use,

FIG. 1.

namely, flat seam and standing seam. In the former method, the sheets of tin are locked into one another at the edges, and nailed to the roof-boards, as shown in Fig. 1. Six or eight 1" wire nails are allowed to the ordinary

sheet. The seams are flattened with wooden mallets and soldered water-tight. The seams constitute the weakest part of a flat tin roof, and should therefore be made and soldered with great care. The tinner should not hurry the soldering, for time is required to properly "sweat" the solder into the seams. Resin is the best flux; chloride of zinc or other acids should not be used, because in a short time they will corrode the tin so badly at the seams as to cause them to leak. A better method of fastening the sheets to the roof is by means of tin cleats about 1 in. X 4 in. These are nailed to the roof, and locked over the upper edge of the sheet, about 15 in. apart.

Standing-seam roofing is that in which the sloping seams are composed of two upstands interlocked, and held in place by cleats. They are not soldered, but are simply locked together, as shown in Fig. 2. The sheets of tin are first double-seamed and soldered together into long strips that reach from eaves to ridge. One edge is turned up about 14 in. and the other about 14 in. The cleats are placed about 15 or 18 in. apart. When the upstands and cleats are locked together the standing seam is about 1 in. high. Only the best quality of tin should be used, and it should be painted on the under side before it is laid. Waterproof roofing felt should be laid under the tin.

FIG. 2.

HEATING AND VENTILATION.

HEAT.

MEASUREMENTS OF HEAT.

Definitions.-Heat is commonly defined as a form of energy due to a rapid vibratory motion of the molecules of which the heated substance is composed, the higher the heat the greater being the rapidity of molecular movement.

Sensible heat is that measurable portion of the heat imparted to a body that serves to raise its temperature, the latter term being used to indicate the intensity of heat or cold.

Instruments that measure the intensity of heat are called thermometers or pyrometers. The quantity of heat developed by the combustion of fuel, or taken up or given off by the gases or vapors in passing from the gaseous to the liquid con. dition, is measured by instruments called calorimeters.

The unit quantity of heat is the quantity required to raise the temperature of 1 lb. of water from 620 to 63°; this unit is called the British thermal unit, abbreviated to B. T. U.

Latent Heat.-The heat expended in changing a body from the solid to the liquid state, or from the liquid to the gaseous state, without change of temperature, is called its latent heat.

The temperature at which a body changes from a solid to a liquid state is called its temperature of fusion; and the number of B. T. U. required to effect this change in a body weighing 1 lb. is called its latent heat of fusion. The temper. ature at which a body changes from a liquid state to a vapor (gas) is called its temperature of vaporization; and the heat required to effect this change in 1 lb. of the liquid is called its latent heat of vaporization.

When a vapor changes back to a liquid, it is said to condense; and when a liquid changes back to a solid, it is said to freeze; in either case, an amount of heat, equal to the latent heat of vaporization or of fusion, as the case may be, must be abstracted from (given up by) the body before the change can be effected.

The following table shows the latent heat of fusion and vaporization for 1 lb. of various substances, they having first been raised to the temperature at which the change takes place, and the pressure being one atmosphere, or 14.7 lb. per sq. in.:

TEMPERATURES AND LATENT HEATS OF FUSION AND OF VAPORIZATION.

The temperature of vaporization in the above table is the boiling point of the liquid under the ordinary atmospheric pressure of 14.7 lb. per sq. in.

Specific Heat.-The specific heat of a body is the ratio between the quantity of heat required to warm that body 1o and the quantity of heat required to warm an equal weight of water 10.

Rule.-To find the number of B. T. U. required to raise the temperature of a body a given number of degrees, multiply the specific heat of the body by its weight, in pounds, and by the number of degrees.

Denote the number of B. T. U. required by U; the specific heat by c; the weight by W; and let t and t1 be the temperatures before and after the heat is applied, respectively.

The specific heat of various substances is shown by the following table:

The table shows that the amount of heat that would be required to raise the temperature of 1 lb. of water would be sufficient to heat, to an equal degree, about 8 lb. of cast iron, or 30 lb. of mercury, or 4 lb. of air, which is about 54 cu. ft.

The specific heats for gases given in the above table are true only when the pressure remains constant.