Glazing.

Glazing by the sq. foot. or light.

Glaziers compute their work by the square foot, in which case the sash is included, by the light. For circular and oval windows take the extreme dimensions, and their product will be the area as windows. computed.

Circular and oval

Roofing.

Slaters and tilers compute the contents of their Area of roofs. work by multiplying the length of the ridge by the girth or "girt" from eave to eave.

In taking the girt for slating the line must be To take the girt for extended under the eaves until it meets the wall, slating; for tiling. but for tiling it is only brought to the eaves.

Double measure is allowed for hips, valleys, gut- Extras.

ters, &c., and no deductions are made for chimneys, skylights, &c., unless expressly stipulated.

The area may be expressed in sq. yds. or squares. Sq. yds.=unit.

Plumbing.

Plumbers' work is estimated by the pound or The lb.=unit. hundredweight (the number of lbs. being expressed.)

Paving.

Pavers' work is done by the square yard.

C

The sq. yd. unit.

SECTION IV.

Specific Gravities.

The Specific Gravity of a body is the ratio of its

Specific Gravity

weight to that of an equivalent volume of distilled defined. water at its maximum density and a barometric pressure of 30 inches.

W

W

Formula, to find the

If, therefore, the weight of a cubic foot of water, which is 1000 ounces, be denoted by w, and that of an equal volume of the body by W, the specific gravity (S. G.) of the latter will be= From this equation we have W=w S. G., or the weight of a cubic foot of any body expressed in ounces, is therefore 1000 times its specific gravity; also if the weight of a cubic foot be known in ounces, the specific gravity is found by dividing by 1000.

weight, knowing

the Specific Gravity

and vice versa.

It is only necessary, therefore, to find the weight General rule. of a volume of water equal to that of the substance whose specific gravity is to be determined. This is done by finding the weight (W) of the body in air and also (W') its weight in distilled water; the loss (W-W') will be the weight of the displaced equal volume of water or w'. Then the weight in air or W divided by w' will be the specific garvity as before.

gravity of a body

As some bodies are lighter than water, or soluble in it, it is necessary, frequently, to employ an auxiliary solid To find the specific whose weights both in air and water are known. lighter than water. This is attached to the lighter body and the compound weighed in air and water. Find the loss in weight of the light body by subtracting that of the heavy body from that of the compound, then divide the weight of the light body in air by this difference for its specific gravity.

The specific gravities of two bodies vary as the To find the specific weights of equal volumes, for S G= and S'G'

W

ως G

gravity of a fluid.

W!

W

dividing one by the other we have

W

S'G' W

hence to find the

specific gravity of a fluid, weigh a body whose S G is known, both in the air and in the fluid. The loss of weight will be that of an equal volume of the fluid, then as the weight of the body (W) is to the loss (W'), so is the specific gravity of the body to that of the fluid.

The annexed table will be found of value, in connection with the formulæ of Section I, in enabling the engineer to compute the weight or volume of various substances. It is arranged alphabetically and compiled from numerous authorities,* that it may be as complete as possible. The weights are computed from the specific gravity by multiplying by 1000 and dividing by 16 to reduce to pounds.

* Chiefly Brande, Jos. P. Cook, Haswell, Hurst, Molesworth, Spon and Trautwine.