To understand how a locomotive works, let us consider how the steam is produced, how it acts on the piston, and how it is controlled. The steam is produced in a locomotive in exactly the same way that steam is produced in a tea-kettle. Now everybody knows that a quart of water in a tea-kettle with a wide bottom placed on a stove will boil more quickly than the same amount of water in a tea-pot with a narrow bottom. The greater the heating-surface--that is, the greater the surface of heated metal in contact with the water--the more quickly the water will boil and the more quickly steam can be produced. In a locomotive the aim is to use as large a heating-surface as possible. This is done by making the fire-box double and allowing the water to circulate in the space between the inner and outer parts, except underneath; also by placing tubes in the boiler through which the heated gases and smoke from the fire must pass. An ordinary locomotive contains two hundred or more of these tubes. The water surrounds these tubes, and is therefore in contact with a very large surface of heated metal. In some engines the water is in the tubes, and the heated gases surround the tubes. The steam as it enters the cylinder should be dry--that is, it should not contain drops of water. This is accomplished by allowing the steam from the boiler to pass into a dome above the boiler. Here the steam, which is nearly dry, enters a steam-pipe leading to the cylinder (Fig. 81). The steam is admitted to the cylinder by means of a slide-valve. From the diagram it can easily be seen that the valve admits steam first on one side of the piston, then on the other. It can also be seen that the valve closes the admission-port, and so cuts off the steam before the piston has made a full stroke. The steam that is shut up in the cylinder continues to expand and act on the piston. At the same time the valve opens the exhaust-port, allowing the steam to escape from the other side of the piston; but it closes this port before the piston has quite finished the stroke. The small quantity of steam thus shut up acts like a cushion to prevent the piston striking the end of the cylinder with too great force. The exhaust-steam escapes through a blast-pipe into the chimney, drives the air before it up the chimney, and thus makes a greater draft of air through the fire-box. This is called the forced draft. The escape of the exhaust-steam causes the puffing of the locomotive just after starting. After the engine is under way the engineer partly shuts off the steam by means of the reversing lever and the puffing is less noticeable. [Illustration: FIG. 81--HOW A LOCOMOTIVE WORKS The arrows show the course of the steam.] The action of the steam may be summed up as follows: 1. Steam admitted to the cylinder (admission). 2. Valve closes admission-port (cut-off). 3. Steam shut up in the cylinder expands, acting on the piston (expansion period).