Question 1. What is the motive power employed in ordinary steam engines?

Answer. The expansive force of steam.

Question 2. How is this expansive force of steam applied?

Answer. It is applied by admitting it into a cylinder (A, fig. 1) in which a piston, B, is fitted so as to move air-tight from one end of the cylinder to the other. The steam, if admitted at C, will force the piston B to the opposite end[2] of the cylinder. When it has reached that end, if the steam is allowed to escape and a fresh supply is admitted to the cylinder through the opening d, it will move the piston back again. In this way, by alternately admitting steam at one end and exhausting it from the other, the piston receives a reciprocating motion, which is communicated to the outside of the cylinder by a rod, R, which is called the piston-rod, which works air-tight through an opening in one of the cylinder-covers, or cylinder-heads, as they are usually called.

Question 3. How is this reciprocating motion of the piston converted into rotary motion?

Answer. By connecting the end of the piston-rod R (fig. 2) by another rod, E; called a connecting-rod, with a crank, P, which is attached to a revolving shaft, S. It is apparent that if the piston B is moved in the direction shown by the dart R, a rotary motion will be given to the crank in the direction of the dart N. When, however, the crank reaches the position shown by the dotted lines in fig. 5, it is plain that a force applied to move the piston in either direction will no longer produce a rotary movement of the crank and shaft. The same thing will occur when the crank is in the opposite position, shown by the full lines. These two positions are called the dead-points of the crank.

Question 4. How is the crank of an ordinary steam engine carried past the dead-points?

Answer. Stationary engines are usually provided with a large and heavy wheel, called a fly-wheel (F F, fig. 2) which is attached to the shaft S. This wheel receives a sufficient amount of momentum from the crank, while the latter is moving from one dead-point to the other, to carry it past those points.

Question 5. How is the steam admitted to and exhausted from the cylinder?

Answer. It is admitted through two channels, c, d, fig. 2, called steam-ways, cast in the cylinder. These ways terminate in a smooth flat surface, f f, called the valve-seat. Their openings in the valve-seat are called steam-ports. Between them is another port or cavity, g, called the exhaust-port, which communicates with the open air. The form of these ports is long and narrow, as shown in fig. 4, which represents a plan of them. Over these ports a valve, V, called a slide-valve, usually made of cast iron, with a cavity, H, on its under side, is fitted so that by moving it backwards or forwards it will alternately cover and uncover the two steam-ports. The valve and valve-seat are inclosed in a sort of box, I I, fig. 2, made of cast iron, called a steam-chest, into which steam is admitted from the boiler by a pipe, J. When the valve is in the position represented in fig. 2, the front steam-port is uncovered and the steam is admitted to the front end of the cylinder, and thus forces the piston towards the back end. If, when the piston reaches the back end, the valve be moved into the position shown in fig. 3, the back steam-port will be uncovered and steam will be admitted to that end of the cylinder. At the same time it will be observed that the aperture of the front steam-port c and that of the exhaust-port are both covered by the cavity in the slide-valve, so that the steam which was admitted to the front end of the cylinder can escape through the steam-port c into the exhaust-port, and thus into the open air. In this way, by moving the valve alternately back and forth, steam is simultaneously admitted first to one end and exhausted from the other, and vice versa.

Question 6. How is the slide-valve moved so as to admit and exhaust the steam at the right time?

Answer. This is done by what is called an eccentric, which is a circular disc, G (fig. 6), the axis of which is not in the centre. The outside of the eccentric is embraced by a metal ring, K K, made in two halves, called an eccentric-strap. The eccentric is attached to the shaft by screws or keys, and revolves with it and inside of the eccentric-strap. To the latter is also attached a rod, L, called an eccentric-rod. It is obvious from fig. 6 that if the eccentric revolves inside of the strap it will impart a reciprocating motion to the rod L. The eccentric, G, strap, K, and rod, L, are represented in fig. 2. Before describing their operation, or rather their connection with the valve V, it is necessary to understand that usually the valve-seat is placed on top of the cylinder, in which position it is difficult to connect the eccentric-rod with the valve. For convenience, therefore, what is called a rocker, r r, is placed between the cylinder and the main shaft of the engine. This rocker has two arms attached to a shaft, s, and the two arms have a vibratory motion about it, as indicated by the dotted lines. The eccentric-rod L is attached by a pin to the lower arm of the rocker, and the valve is connected to the upper arm by the rod M, called the valve-stem, or valve-rod. It is obvious that as the shaft S and eccentric G revolve, a reciprocating or vibratory motion will be given to the rocker, which will be communicated to the valve by the valve-stem; and it is only necessary to fix the eccentric in the proper position on the shaft, in relation to the crank and piston, to give the valve the required motion for admitting and exhausting the steam to and from the cylinder at the right time.