AIR-PLANT, more properly epiihyte, a Plant growing upon the trunk or top of another Plant (the host), but not anatomically connected with it. Air-plants derive their nutriment chiefly from the air and rain, commonly by means of aerial roots. Parasitic plants, on the other hand, are directly connected with the plant upon which they grow and extract their food from the host. Air-plants are most com mon in moist tropical forests, often completely covering the trunks and branches of trees. They belong chiefly to the ferns, mosses, brome liads, orchids and aroids, and are mostly her baceous, but sometimes shrubs are epiphytic upon tropical trees. In the United States air plants are found chiefly in Florida, but one, A machine by means of which air or other gas, water or other liquids may be removed from an enclosed space. The essential part is a hollow cylinder, in which an air-tight piston is made to move up and down by a rod. From the bottom of the cylinder a connecting tube leads to the space which is to be exhausted, which is usually formed by plac ing a bell-glass, called the receiver, with edges ground smooth and smeared with lard, on a flat, smooth plate or table. When the piston is at the bottom of the barrel, and is then drawn up, it lifts out the air from the barrel, and a portion of the air under the receiver, by its own expansive force, passes through the con necting tube and occupies the space below the piston, which would otherwise be a vacuum. The air in the receiver and barrel is thus rare fied. The piston is now forced down, and the effect of this is to close a- valve placed at the mouth of the connecting tube and opening in ward into the barrel. The air in the barrel is thus cut off from returning into the receiver, and, as it becomes condensed, forces up a valve in the piston, which opens outward, and thus escapes into the atmosphere. When the piston reaches the bottom and begins to ascend again, this valve closes; and the same process is re peated as at the first ascent. Each stroke thus
diminishes the quantity of air in the receiver; but from the nature of the process it is evident that the exhaustion can never be complete. Even theoretically there must always be a por tion left, though that portion may be rendered less than any assignable quantity; and prac tically the process is limited by the elastic force of the remaining air being no longer sufficient to open the valves. The degree of rarefaction is indicated by a gauge on the principle of the barometer. By means of the partial vacuum formed by the air-pump a great many inter esting experiments can be performed, illustrat ing the effects of atmospheric pressure and other mechanical properties of gases. The air pump was invented by Otto Guericke, in 1654; and though many improvements and varieties of structure have been since devised, the prin ciple of all is the same. Two barrels are gen erally used, so as to double the effect of one stroke. In some air-pumps, stop-cocks turned by the hand take the place of valves, and in others the entrance of the connecting tube into the cylinder is so contrived that the valve through the piston is not required. The air pump used in connection with the densimeter is of the ordinary construction.
The use of compressed air for pumping water, oil or other liquids from wells, vats and tanks, is now general. The simplicity and ease of maintenance attending its use make it far superior to any other means of conveying liquids. In deep wells particularly, great economy is found in the use of air as compared with the old time deep-well pump. No valves, no reciprocating rods and plungers, and no wasteful steam cylinder at the top to operate them are required. The air may be compressed at any convenient point, such as an existing power house, and yet do pumping at an isolated or distant place; several separate wells, if neces sary, being pumped from one central station.