One of the most important steps in the development of the steam engine was the addition of the cut-off. Prior to its adoption steam was admitted to the cylinder during the whole time the piston was making its stroke from one end of the cylinder to the other. In the cut-off (see Fig. 79), when steam is being admitted through the port _p_, and the piston is being driven in the direction of the arrow, it was found that if the steam were cut off when the piston arrived at the position 1, the expansive action of the steam behind it in chamber _a_ would continue to carry the piston with an effective force to the end of its stroke, or to position 2. This of course effected a great saving in steam. Various cut-offs have been devised. Perhaps that most easily recognized by most persons is the one seen in the engine room of our side wheel steamers, of which illustration is given in Fig. 80. This was invented in 1841 by F. E. Sickels, and was the first successful drop cut-off. It was covered by his patents, May 20, 1842, July 20, 1843, October 19, 1844, No. 3,802, and September 19, 1845, No. 4,201. A rock shaft _s_ is worked by an eccentric rod _e_ from the paddle wheel shaft. The rock shaft has lifting arms _a_ that act upon and alternately raise the feet _c_ on rods _b b_. One of these rods _b_ works the valves that admit steam, and the other the valves that discharge steam. The valve rod that admits steam has a quick drop, or fall, to cut off the live steam before the piston reaches the end of its stroke. In Fig. 81 is shown the celebrated Corliss cut-off and valve gear, in which a central wrist plate and four radiating rods work the valves. This valve gear was covered in Corliss patents, No. 6,162, March 10, 1849, and No. 8.253, July 29, 1851. [Illustration: FIG. 80.--SICKELS’ DROP CUT-OFF VALVE GEAR.] [Illustration: FIG. 81.--CORLISS CUT-OFF AND VALVE GEAR.]