[Illustration: Utility combination pump governor.]
_A condenser_ is an apparatus, separate from the cylinder, in which exhaust steam is condensed by the action of cold water; _condensation_ is the act or process of reducing, _by depression of temperature or increase of pressure_, etc., to another and denser form, as gas to the condition of a liquid or steam to water. There is an electrical device called “a condenser” which must not be confounded with the hydraulic apparatus of the same name; there is also an optical instrument designated by the same term, which belongs to still another division of practical science.
_A vacuum_ is defined very properly as an empty space; a space in which there is neither steam, water or air—the absolute absence of everything. The condenser is the apparatus by which, through the cooling of the steam by means of cold water, a vacuum is obtained.
_The steam after expelling the air from the condenser fills it with its own volume_ which is at atmospheric pressure nearly 1700 times that of the same weight of water.
Now when a vessel is filled with steam at atmospheric pressure, and this steam is cooled by external application of cold water, it will immediately give up its heat, which will pass off in the cooling water, and _the steam will again appear in a liquid state_, occupying only 1/1700 part of its original volume.
But if the vessel be perfectly tight and none of the outside air can enter, the space in the vessel not occupied by the water contains nothing, as before stated. The air exerting a pressure of nearly 15 pounds to the square inch of the surface of the vessel tries to collapse it; now if we take a cylinder fitted with a piston and connect its closed end to this vessel by means of a pipe, the atmospheric pressure will push this piston down. The old low pressure engines were operated almost entirely upon this principle, the steam only served to push the piston up and exhaust the air from the cylinder.
In Fig. 578 is exhibited the effect of jets of water from a spray nozzle meeting a jet of steam; the latter instead of filling the space with steam is returned to its original condition of water and the space as shown becomes a vacuum.