This is a variable cut-off, which will be understood by the following description and accompanying engravings, in which Fig. 1 is an interior view of the steam chest of a steam engine, with a side view of the slide, cut-off, and mechanism for operating the latter ; Fig. 2 is a longitudinal section of the slide valve, and its seat with a corresponding section of the cut-off ; Fig. 3 is a perspective view of one of the seats of the cut-off valves, and Fig. 4 is a perspective view of one of the rings which are attached to the cut-off valves for the purpose of operating them. A is the steam chest, B is a slide valve of the well-known kind, for the induction and eduction of the steam to and from the cylinder, working on a seat, a a, and moved by an eccentric on the crank shaft of the engine ; b V are the steam ports, and c c' the exhaust ports in the valve seat, the former communicating with the cylinder, and the latter with the exhaust pipe ; d d' are the induction pas-sages through the valve, and c e' are eduction cavities for forming communication between the ports, b and c and between b' and c '. D D' is a sliding box containing two steam chambers, D and D', and fitted to a slide on the back of the main valve, B ; these chambers being separate from each other, and open on the side next the slide valve to communicate respectively with the induction passages, d d', of the main valve, but being closed to the steam chest, A, except through the two hollow plug cut-off valves, E E', whose seats, /"./"extend all the way through their respective steam chambers, so as to admit steam at all times to the interiors of the cut-off valves, E E'. These valves have each several very narrow openings, g g, in their sides, to correspond with the same number of openings, j j, in their respective seats, to admit steam from the chest, A, to the chambers, 13 D', to be supplied through the passages, d d', to the cylinder, and to cut it off by a very slight movement circularly in their seats. The two-chambered sliding valve box, D D', is connected with one eccentric on the crank shaft, and from the movement of this derives the necessary motion for their operation. The manner in which these valves are operated is as follows :—At one end of each valve is secured a collar, h h', on which there is a projection, i (Fig. 4), which, from its form, may be termed a toe and heelpiece ; and inside the steam chest, on each end of the valve, is secured a piece, 11', these are placed in such a position that the toes, i i', will strike them, as the sliding valve box and piston arrive near the end of their stroke in either direction, and thus cause the cut-off valves to be opened in their proper turn by the time the piston completes its stroke ready to admit the steam to the cylinder as the slide valve, B, begins to open the port b or b'. On the same side of the valve as i i' are two bar G G', each having shoulders, h h', so arranged that the heels of the projections, i i', will strike them after the piston hao performed a certain portion of its stroke, and thus cause the cut-off valves to be closed in their proper turn. The cutting off is caused to take place sooner or later in the stroke by changing the position of the bars, G G', longitudinally, to throw i lc' farther apart or bring them closer together, and to admit of this they are attached to a stationary stand, I, by means of bolts, m m', passing througli slots, c e',.and provided with pins, n n', working in slots, pp', in the stand. The shifting of the bars is effected by a small shaft, q, passing through a stuffing-box in the steam chest, and having a pinion, r, on it, gearing into toothed racks on G G'. It will be seen that the cut-off valves being hollow and receiving steam at the interior, and having opsnings at opposite points in themselves and their seats, must receive an ( equal pressure of steam on all sides, or in | other words, they are balanced laterally, but owing to their taper form there is a slight pressure in a longitudinal direction tending to force them into their seats. To prevent the friction that would be produced by this pressure in turning the valves, they are caused to move a slight distance longitu- dinally out of their seats, by making the ends, //', project out of the valve box, D D', with a spiral inclination (the form of a single turn of a screw thread) as shown at s s, Fig. 3, and making the inner faces of the collars, h h', of corresponding form, as seen at t, Fig. 4. The jogs, a u, on the seats, f f, and collars, h h', serve as stops to prevent the turning of the cut-off valves further than is necessary in closing. This cut-off prevents the waste of much steam, and consequently saves fuel. It is the invention of Addison Crosby, of Fredonia, N. Y., and was patented Jan. 19, 1858, from whom all further information can be obtained. A notice of it will be found on page 166 of the present volume of the SCIENTIFIC AMERICAN.