EXAMPLE. Required the length of feed pipe, capable of supplying a boiler with water when the pressure of steam is 4 pounds per square inch.

2.5 x 4 10 feet above the surface of the water in the boiler.

STEAM ENGINE is the name of a machine which derives its moving power from the elasticity and condensibility of the steam of boiling water, and from repeated observations taken from engines working at maximum, the following table is derived.

N.B. These are to be considered as the velocities of land engines, or engines whose connecting rods are not less than 3 times the length of stroke; but marine engines, being generally confined to connecting rods of not more than 2 or 24 times the length of stroke, have their maximum velocities considerably reduced. Hence, the following table will be found pretty correct when the periphry of the wheels move with a velocity of about 1300 feet per minute, and the floats, or paddle boards, calculated by the following rule, which I have found, in practice, to produce the greatest satisfaction, namely, economising of fuel, a steady supply of steam, without waste, and the vessel propelled quicker than when the surface of the floats were less and moving at a greater velocity.

To find the Surface of the Floats.

RULE.-Multiply the number of horses'. power, that the engine is equal to, by 3.5, divide the product by the diameter of the wheel, in feet, and the quotient is the area of each float, or paddle board.

EXAMPLE.-Required the area of each paddle board for an engine of 65-horse power, with wheels of 18 feet diameter.

Suppose the float to be 7 feet long, then 12.6 ÷ 7 = 1.8 feet, the breadth of each board nearly.

And when there is only one engine in the vessel, of the quotient is the area of each board nearly.

Each wheel, from 12 to 14 feet diameter, ought to have 12 floats; from 14 to 16 feet diameter, 14 floats; from 16 to 18 feet diameter, 16 floats; and from 18 to 22 feet diameter, 18 floats, &c.

Foundation of the Rule for Calculating the Power of a Steam Engine.

1. In estimating the power or effect of an engine, we generally reckon the force of steam, in the boiler,

24 lbs. per square inch; although, in practice, it is customary to load the safety valve with 3 or 34 lbs., according to the condition of the boiler.

2. We suppose the barometer or vacuum gauge to indicate 26 inches on an average; of course, the benefit derived from the condenser will be nearly 13 lbs., leaving the piston with a force equal to 15 lbs; then, allowing for friction, and for changing from a reciprocating to a circular motion, leaves only about 7 lbs. effective power in giving motion to machinery. Hence the rule:

1.-Multiply a horse power, namely, 33,000 lbs. by the number of horses' power required, and divide by the number of feet the piston travels per minute, multiplied by the effective pressure 7.25 lbs.; and the quotient will be the cylinder's area.

2.—Multiply the area of cylinder by the effective pressure, and by the number of feet the piston travels per minute, and divide by 33,000, the quotient will be the number of horses' power.

An Approximate Table for Calculating the Power of an Engine.

RULE 1-Multiply the decimals in opposite the length of stroke, by the horses' power, and the product will be cylinder.

the

2. Multiply any cylinder's area by the decimals in column 2, opposite the length of str product will be the number of horses

column 1, number of the area of

A Table of Gauge Points for finding the Power of an Engine by the Engineer's Slide Rule.

RULE. Set the gauge point upon C to 1 upon D, and against the number of horses' power upon C, is the diameter in inches upon D; Or, against the diameter in inches upon D, is the number of horses' power upon C.

EXAMPLE 1.-What diameter must a cylinder be with a 4 feet stroke, to be equal to 20 horses' power? 33000 x 20 660000 455.172 inches area,

=

200 × 7.25 1450

or 24.1 inches diameter nearly.

By the Table.

Opposite a 4 feet stroke in column 1 is 22.76; and 22.76 × 20 = 455.2 inches area of cylinder.

By the Slide Rule.

Set 343 upon C to 1 upon D; and against 20 C is 24.2 inches diameter upon D. upon

EXAMPLE 2.-What number of horses' power will an engine be equal to, when the cylinder's diameter is 19 inches and stroke 3 feet?

192 x .7854 × 7.25 x 192

33000

11.96 or 12 horses' power nearly.

394672.7328

33000

By the Table.

192 × .7854 = 283.5294.-And opposite a 3 feet stroke in column 2 is .0422. Then 283.5294 × .0422 11.9649 horses' power.

By the Slide Rule.

Set 33 upon C to 1 upon D, and against 19 upon D is 12 horses' power nearly, upon C.

The Steam Way.

Multiply any cylinder's area by .034; and the product will be the area of port or steam way.

EXAMPLE.-What area of port or steam way is necessary for a cylinder 36 inches diameter ?

36 inches diameter 1017.8 inches area > .034 34.6 inches area of steam way.

The Air Pump.

The Air Pump for a land engine generally requires to be larger in proportion to the cylinder, than the air pump for a marine engine, on account of having frequently to condense with water at a higher temperature, hence, when the stroke of the bucket is half the stroke of the piston, multiply the cylinder's diameter in inches by .67, and the product is the diameter of air pump.-Again, multiply the diameter of the cylinder of a marine engine, in inches, by .575, and the product is the diameter of air pump.

EXAMPLE.-What diameter of air pump is requisite for an engine whose cylinder is 28 inches diameter ?

28 × .67 = 18.76 inches diameter.

M