The best known and most generally used class of stationary engines at the present time is, however, that which has the so-called "drop cut-off," or "detachable valve-gear." The oldest well-known form of valve-motion of this description now in use is that known as the Sickels cut-off, patented by Frederick E. Sickels, an American mechanic, about the year 1841, and also built by Hogg, of New York, who placed it upon the engine of the steamer South America. The invention is claimed for both Hogg and Sickels. It was introduced by the inventor in a form which especially adapted it to use with the beam-engine used on the Eastern waters of the United States, and was adapted to stationary engines by Messrs. Thurston, Greene & Co., of Providence, R. I., who made use of it for some years before any other form of "drop cut-off" came into general use. The Sickels cut-off consisted of a set of steam-valves, usually independent of the exhaust-valves, and each raised by a catch, which could be thrown out, at the proper moment, by a wedge with which it came in contact as it rose with the opening valve. This wedge, or other equivalent device, was so adjusted that the valve should be detached and fall to its seat when the piston reached that point in its movement, after taking steam, at which expansion was to commence. From this point, no steam entering the cylinder, the piston was impelled by the expanding vapor. The valve was usually the double-poppet. Sickels subsequently invented what was called the "beam-motion," to detach the valve at any point in the stroke. As at first arranged, the valve could only be detached during the earlier half-stroke, since at mid-stroke the direction of motion of the eccentric rod was reversed and the valve began to descend. By introducing a "wiper" having a motion transverse to that of the valve and its catch, and by giving this wiper a motion coincident with that of the piston by connecting it with the beam or other part of the engine moving with the piston, he obtained a kinematic combination which permitted the valve to be detached at any point in the stroke, adding a very simple contrivance which enabled the attendant to set the wiper so that it should strike the catch at any time during the forward movement of the "beam-motion."