RULE.-Find, by the rules in the "Mensuration of Solids," the solidity of the material of which the weight is required, and multiply that solidity by the factor, in the foregoing table.

EXAMPLE 1.-Required the weight of a bar of iron 12 feet long and 1 inch square?

Weight of 1 inch bar-iron, as per table, page 135, is 0.281. 12 feet = 144 inches.

Then 281 X 144 inches = = 40·464, or 40, or 40 lbs. nearly. EXAMPLE 2.-Required the weight of a plank of yellow fir, 11 inches wide, 3 inches thick, and 20 feet long?

20 feet = 240 inches.

=

0.023; therefore

One cubic inch yellow fir, as per table, 023 × 240 × 11 × 3 = 182.160, or 182%, or 1824 lbs. nearly.

THE STEAM-ENGINE.

THE power of the steam-engine is estimated by that exerted by the horse. A horse-power, as fixed by Watt, is equal to 33,000 lbs. avoirdupois, raised one foot high per minute; and one day's work of a horse, is this power, acting through eight hours. The pressure of steam is calculated in pounds avoirdupois on the square inch, in perpendicular inches of mercury, and in atmospheres; each atmosphere is estimated as equal to the average pressure of our atmosphere at the surface of the earth. The pressure of the atmosphere is reckoned as equal to that of 30 perpendicular inches of mercury; or 14·7 lbs. per square inch, or 11.55 lbs. per circular inch.

To find the Horses' power of an Engine, according to Mr. Watt's rule.

From the diameter of the cylinder in inches, subtract 1, square the remainder, multiply the square by the velocity of the piston in feet per minute, and divide the product by 5640. The quotient will be the number required.

GENERAL PROPORTIONS OF CONDENSING ENGINES.

Cylinder. The best proportion is when the length is twice the diameter; because the cooling

surface is then least, in proportion to the content of steam.

Air-Pump and Condenser.-In double condensing engines, these are made, by Boulton and Watt's rule, each to measure one-eighth the content of the cylinder.

Velocity of the Piston to produce the best effect.In engines working the steam expansively, 100 times the square root of the length of the stroke in feet, is the best velocity in feet per minute.

In engines not working expansively, 103 times the square root of the length of the stroke in feet, is the best velocity in feet per minute.

To find the quantity of Water required for Steam and Injection.-Multiply the area of the cylinder in feet, by half the velocity in feet for single, and by the whole velocity in feet for double engines. Add

for cooling and wasting; and this, divided by 1497, (at the common pressure on the valve of 2 lbs. per circular inch,) will give the quantity of water required for steam per minute.

The quantity of water for injection should be 24 times that required for steam.

The diameter of the injection-pipe should be th part of that of the cylinder.

The valves should be as large as practicable. The boiler should be capable of evaporating about 12 gallons per hour for each horse-power.