_Saturated steam_ is that in contact with water at the same temperature; saturated steam is always at its condensing point, which is always the boiling point of the water, with which it is in contact; in this it differs from superheated steam. _Superheated steam_, also called steam-gas, is steam dried with heat applied after it has left the boiler. _Total heat of steam_ is the same as steam heat. _Wet steam_, steam holding water mechanically suspended, the water being in the form of spray. Specific gravity of steam is ·625 as compared to air under the same pressure. The properties which make it so valuable are: 1. The ease with which we can condense it. 2. Its great expansive power. 3. The small space in which it shrinks when it is condensed either in a vacuum chamber or the air. A cubic inch of water turned into steam at the pressure of the atmosphere will expand into 1,669 cubic inches. THE DAVIDSON. The Davidson pump is shown complete in Fig. 248; the valve motion consists principally of _a valve, valve pistons, valve pin and cam_. The main valve is operated by a positive mechanical connection between it and the main piston rod, also by the action of steam on the valve pistons. The engraving, Fig. 249, shows the details of valve gear and steam cylinder. The steam end consists of the cylinder, M, valve, A, and valve pistons, B and B. These pistons are connected with sufficient space between them for the valve, A, covering the steam ports, F and F_{1}, Fig. 250. [Illustration: FIG. 248.] The valve is operated by the steel cam, C, acting on a steel pin, D, which passes through the valve into the exhaust port, N, in which the cam is located. In addition to this positive motion steam is alternately admitted to and exhausted from the ends of the valve piston through the ports, E and E_{1}, which moves the pistons, B and B_{1}. [Illustration: FIG. 249.] Assumed that this pump is at rest with the valve, A, covering the main steam ports, F and F_{1}, in which position the cam C, holds the main valve by means of the valve pin, D, so that ports, E and E_{1}, admit steam to one end of the valve piston at the same time connects the other end with the exhaust port. The steam, acting on the valve pistons, moves both, opening the main ports, F and F_{1}, admitting steam to one end of the steam cylinder and opening the other end to the exhaust. If the valve occupies any other position than the one described, the main ports, F and F_{1}, will be opened for the admission and exhaust of steam; consequently it is evident that this pump will start from all points of the stroke. [Illustration: FIG. 250.]