Stephenson’s son Robert is commonly credited with the invention of an ingenious link motion, although the invention is also claimed for W. T. James of New York. We must pause a moment for a description of this ingenious link motion, because it became a standard in locomotive construction that is still in service. As explained in Chapter IX steam should be used expansively in order to obtain a maximum of efficiency. After a certain amount of steam has been admitted into the cylinder it is cut off from the boiler and it pushes the piston by its own expansion. The speed of the locomotive must be varied by varying the point of cut-off, and the direction of motion of the locomotive is reversed by reversing the motion of the valve. In the Stephenson link motion this is all effected by the operation of a single lever. Fig. 61 shows the arrangement. The valve is moved back and forth by the rod _A_ which is connected to the arm _B_ suspended from the frame of the locomotive. The drive shaft is shown at _C_ and it carries two eccentrics projecting on opposite sides of the shaft which move the rods _D_ and _E_ back and forth. The outer ends of these rods are connected to opposite ends of a curved link _F_ and a pin _G_ on the arm _B_ engages a slot in the link. The link is held up by a rod _H_. As the drive shaft rotates the slotted link is oscillated back and forth on its own center. When the link is lowered so that the pin _G_ is at the center of the slot there is no motion of the valve rod _A_, but when the pin is at the top or at the bottom of the slot the valve rod has its greatest motion. As the top of the link is moving in one direction while the bottom is moving in the other direction, it will be plain that the action of the valve when _G_ is at the top of the link will be the reverse of the action when _G_ is at the bottom. The link is raised and lowered by means of a lever _I_ which is connected to one arm of a T-shaped lever _J_ which on one side of its fulcrum is connected to the rod _H_ and on the other to the counterweight _K_. There are certain defects in the Stephenson link motion that we cannot discuss here. These became serious as engines grew larger and more powerful so that to-day it has been largely superseded by other valve gears. The most important of these is the Walschaerts gear, invented by a Belgian engineer. This gear, unlike Stephenson’s, is conspicuously placed outside the drivers and is particularly noticeable because of its peculiar grasshopper motion. The action of the gear is so complicated that a description of it would be out of place in this book. MODERN LOCOMOTIVES