ANALYSIS. The motor element of the engine is the cylinder and piston (Fig. 2). The cylin der, C, is a hollow cylinder of metal closed at on a water pipe, A, connected with a cistern, C'. This condensed the steam in the cylinder, and caused a vacuum below the piston, which was then forced down by the pressure of the atmos phere, bringing with it the end of the beam to which it was attached, and raising the other along with the pump-rod. The cock was then turned to admit fresh steam below the piston, which was raised by the counterpoise; and thus the motion began anew. The opening and shut ting of the cocks was at first performed by an attendant, hut in 1713 a boy named Humphrey Potter devised a system of strings and levers by which the engine was made to work its own valves. In 1717 Henry Beighton invented a simpler and more scientific system of 'hand gear,' which rendered the engine completely self acting.
The next essential improvements on the steam engine were those of Watt. The first and most important improvement made by Watt was the separate condenser, patented in 1769. He had observed that the jet of cold water thrown into the cylinder to condense the steam necessarily reduced the temperature of the cylinder so much that a great deal of the steam flowing in at each upward stroke of the piston was condensed be fore the cylinder got back the heat abstracted from it by the spurt of cold water used for con densing the steam in the cylinder. The loss of steam arising from this was so great that only about one-fourth of what admitted into the cylinder was actually available as motive power. Watt, therefore, provided a separate vessel in which to condense the steam, and which could lss kept constantly in a state of vacuum, with out the loss which arose when the cylinder itself both ends, except for small openings for the en trance and escape of steam and for the passage of the piston rod. Inside the cylinder is the piston, P, a circular disk of metal fitting steam tight and capable of movement lengthwise of the cylinder. The openings S, and S, provide for the admission of steam into the cylinder, and the openings E, and E, provide for the exhaust of steam from the cylinder. These passages are opened and closed by valves. These valves oper ate in pairs; when valves S, and E, are open valves and E, are closed and vice versa. If steam be admitted by opening valve 5, its pres sure forces the piston P to the opposite end of the cylinder. Valves S, and E, now close and
valves and E open, and steam entering at S, forces the piston to return and press, as it advances, the previous cylinderful of steam out of the exhaust port E,. The repeated and alter nate opening and closing of the two pairs of valves, as described. causes the piston to recip rocate back and forth in the cylinder. This motion is carried outside of the cylinder by means of the piston rod II. This is a cylindrical rod attached rigidly to the piston and passing out of the cylinder through a steam-tight orifice in the cylinder head. At its outer end the pis ton, rod is attached to a rectangular piece H, called the crosshead, which slides between two guides, 0G. To the opposite end of the cross head is hinged the rod S, called the connecting rod, the forward end of which is journaled to a crank K, which operates the fly-wheel \V.
In the steam engine, as actually constructed, these different parts and their movements are variously modified, but the essential operating parts of all reciprocating steam engines are: The cylinder and piston; the valves and valve gear; the piston rod; the crosshead and guides; the connecting rod; the crank and crank-shaft or fly-wheel.
The ordinary unit of measure of the work done by a steam engine is the horse power, and it was originated by Watt. As defined by Watt a horse power is the work done in lifting 33,000 pounds one foot high in one minute. Now if the area of the piston P, Fig. 2, is w square inches and the pressure of the steam admitted to the cylinder is y pounds per square inch, then the pressure exerted by the steam against the pis ton is w X y. If now the total travel or stroke of the piston is s feet, then the foot pounds of work for such stroke is represented by w X y X s, and if the piston makes 7i strokes per minute, the foot pounds of work of the piston per min ute are represented by u- X y Xs X n. This amount, divided by 33.000, gives the horse power of the engine. Assuming all of the other factors to remain the same or to be constant, the horse power of an engine may be varied by vary ing either the area of the piston, the length of the stroke, the pressure of the steam, or the number of strokes per minute. The whole of the theoretical horse power, however, is never avail able for useful work, owing to the friction of the moving parts and other causes.