The link motion has been claimed both for the younger Stephenson and W. T. James, of New York, the latter being probably its real inventor. Its purpose is to reverse the engine and also to cut off steam in either direction, so that it may act expansively. The form of link motion most generally used is shown in Fig. 103, and is known as Stephenson’s. A B are two eccentrics projecting in opposite directions from the center of the common drive shaft, their rods being connected at their outer ends by a curved and slotted link C D. In the slot of this link plays a pin E, carried by a pendent swinging lever G F, which lever is jointed at its lower end to the slide valve rod H. A T-shaped lever I L K M has one arm at I connected by a rod with the slotted link at C. The opposite arm is provided with a counter weight at K to balance the weight of the link C D and eccentric rods, and the upright arm is connected at M to a rod operated by a hand lever P within easy access of the engineer. When the link C D is lowered the eccentric B imparts its throw to pendent lever G F and valve rod H, and the eccentric A will only swing the end C of the link without imparting any effect to the valve. When link C D is drawn up so that pin E is in the bottom of the slot, the eccentric A is active and B inactive, and as A has an opposite throw to B, the action of the valve is reversed. If link C D be drawn half way up, the pin E becomes the center of the oscillation of the link, and the valve rod is not moved at all. By adjusting the link nearer to or further from the central position, the throw of the slide valve may be made shorter or longer, and the steam cut off at a later or earlier period in the stroke of the piston. [Illustration: FIG. 104.--LOCOMOTIVE ENGINE NO. 999.] Fig. 104 is a type of the best modern express locomotive. This is the famous 999 of the New York Central & Hudson River Railroad. Its cylinders are 19 × 24 inches, driving wheels 86½ inches in diameter, weight 62 tons, steam pressure 190 pounds. This engine hauls the Empire State Express at a speed of 64.22 miles an hour, excluding stops, or more than a mile a minute. [Illustration: FIG. 105.--COMPOUND LOCOMOTIVE.] In securing a higher efficiency and a greater economy in the use of steam, the most recent developments in the locomotive have been in the application of the principle of the compound expansion engine, in which two or more cylinders of different diameters are used, the steam at high pressure acting in the smaller cylinder, and being then exhausted into and acting expansively upon the piston of the larger cylinder. A fine example of the compound locomotive is shown in Fig. 105. The cylinders are arranged in pairs, the small high pressure cylinder above, and the larger low pressure cylinder below, both piston rods engaging a common cross head. The application of this principle of the compound engine is said to involve a saving in coal of over 25 per cent.