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 × 144 inches 40·464, or 40,5 40.464, or 4015, 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.

One cubic inch yellow fir, as per table,

0.023; therefore

·-023 × 240 × 11 × 3 = 182∙160, or 182,2 182.160, or 1826, or 1821 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