So far as locomotives are concerned, fast time, especially with heavy trains, is generally dependent more upon the supply of steam than it is on the size of the wheels. Without steam to turn them, big wheels are useless; but with an abundant supply there is no difficulty in turning small wheels at a lively rate. Speed, therefore, is to a great extent a question of boiler capacity, and the general maxim has been formulated that "within the limits of weight and space to which a locomotive boiler must be confined, it cannot be made too big." But the maximum speed at which a locomotive can run when an adequate supply of steam is provided also depends on the perfection of the machinery. At 60 miles an hour a driving-wheel 5½ feet in diameter revolves five times every second. The reciprocating parts of each cylinder of a Pennsylvania Railroad passenger engine, including one piston, piston-rod, cross-head, and connecting rod, weigh about 650 pounds. These parts must move back and forth a distance equal to the stroke, usually two feet, every time the wheel revolves, or in a fifth of a second. It starts from a state of rest at each end of the stroke of the piston and must acquire a velocity of 32 feet per second, in one-twentieth of a second, and must be brought to a state of rest in the same period of time. A piston 18 inches in diameter has an area of 254½ square inches. Steam of 150 pounds pressure per square inch would therefore exert a force on the piston equal to 38,175 pounds. This force is applied alternately on each side of the piston, ten times in a second. The control of such forces requires mechanism which works with the utmost precision and with absolute certainty, and it is for this reason that the speed and the economical working of a locomotive depend so much on the proportions of the valves and the "valve-gear" by which the "distribution" of steam in the cylinders is controlled. [Illustration: Fig. 35.--Interior of a Round-house.] The engraving (Fig. 36) on p. 133 represents the cab end of a locomotive of the New York Central & Hudson River Railroad, looking forward from the tender, and shows the attachments by which the engineer works the engine.[12] This gives an idea of the number of keys on which he has to play in running such a machine. There is room here for little more than an enumeration of the parts which are numbered: 1. Engine-bell rope. 2. Train-bell rope. 3. Train-bell or gong. 4. Lever for blowing whistle. 5. Steam-gauge to indicate pressure in boiler. 6. Steam-gauge lamp to illuminate face of gauge. 7. Pressure-gauge for air-brake; to show pressure in air-reservoirs. 8. Valve to admit steam to air-brake pump. 9. Automatic lubricator for oiling main valves. 10. Cock for admitting steam to lubricator. 11. Handle for opening valves in sand-box to sand the rails. 12. Handle for opening the cocks which drain the water from the cylinders.