_To set the valve_ place the piston in the mid-stroke position and set the auxiliary valve with rocker arm plumb, and the preliminary valve covering all its ports equally. These positions may be secured by adjusting the position of the clamp on the valve stem and move the main piston sufficiently so that the auxiliary valve will open one of its ports. THE BLAKESLEE. [Illustration: FIG. 264.] The accompanying engraving, Fig. 264, shows a section of the slide valve chamber, which is fitted to the top of cylinder. The slide valve, B, is fitted in the valve chamber, A, the cut-off valve, C, works on top of the slide valve and is operated by the valve stem, D. The lever, F, which moves the valve, is attached to the crosshead on the piston rod. While the crosshead moves the length of the stroke, the cut-off valve, C, moves twice the width of the steam ports in the cylinder. The opening in the cut-off valve, C, is equal to the length of the dog, E, plus the throw of the slide valve, B, plus one-sixteenth inch. The length of the valve chamber on the inside is equal to the length of the cut-off valve minus the throw of the cut-off valve. The valve stem, D, is connected to the cut-off valve, C, so that it moves the latter valve a distance proportional to the movement of the steam piston. The opening through the cut-off valve is equal in width to the distance from inside to inside of the ports on top of the slide valve. As the piston moves to the right, as shown in the drawing, the cut-off valve moves to the left, so as to open the right-hand end of the valve box and admits steam, which acts upon the slide valve and moves it to the left-hand end of the valve box. This opens the port wide in the slide valve, which corresponds with the port leading to the cylinder while in the upper side of the slide valve the port is nearly closed by the cut-off valve, so that little steam is admitted to the cylinder; consequently, the piston is gradually started on its return stroke. On the return stroke the cut-off valve begins to move and opens one of the ports on top of slide valve, which stands still until the piston has made a half stroke. By this time the valve is wide open “full port” to the cylinder. The cut-off valve, C, coming in contact with the dog, E, both the slide valve and cut-off valve move together to the end of the stroke. Notice that as soon as the cut-off valve, C, engages the slide valve and it begins to move, the port to the cylinder closes so that at the end of the stroke but little steam can get to the piston. The cut-off valve opens the valve box, A, to admit steam to complete the stroke of slide valve, and should the steam fail to throw the slide valve, the piston, by means of the cut-off valve, C, coming in contact with the dog, E, moves the slide valve, thereby opening the opposite port and preventing the piston from striking the cylinder head.