The cylinder is represented at C (fig. 12.)--in which the piston P moves steam-tight. It is closed at the top, and the piston-rod being very accurately turned, runs in a steam-tight collar B furnished with a stuffing-box, and constantly supplied with melted tallow or wax. Through a funnel in the top of the cylinder, melted grease flows upon the piston so as to maintain it steam-tight. Two boxes A A, containing the valves for admitting and withdrawing the steam, connected by a tube of communication T, are attached to the cylinder; the action of these valves will be presently described. Below the cylinder, placed in a cistern of cold water, is a close cylindrical vessel D, called the condenser, communicating with the cylinder by a tube T´, leading to the lower valve-box A. In the side of this condenser is inserted a tube, the inner end of which is pierced with holes like the rose of a watering-pot; and a cock E in the cold cistern is placed on the outside, through which, when open, the water passing, rises in a jet on the inside. The tube S, which conducts steam from the boiler, enters the top of the upper valve-box at F. Immediately under it is placed a valve G, which is opened and closed by a lever or rod G´. This valve, when open, admits steam to the top of the piston, and also to the tube T, which communicates between the two valve-boxes, and when closed suspends the admission of steam. There are two valves in the lower box, one H in the top worked by the lever H´, and one I in the bottom worked by the lever I´. The valve H, when open, admits steam to pass from the cylinder _above_ the piston, by the tube T, to the cylinder _below_ the piston, the valve I being supposed in this case to be closed. This valve I, when open, (the valve H being closed,) admits steam to pass from below the cylinder through T´ to the condenser. This steam, entering the condenser, meets the jet, admitted to play by the valve E, and is condensed. The valve G is called the _upper steam valve_; H, _lower steam valve_; I, the _exhausting-valve_; and E, the _condensing valve_. Let us now consider how these valves must be worked in order to produce the alternate ascent and descent of the piston.