Machines for raising water and other fluids; usually consisting of a tube or tubes, in which valves and pistons, or buckets, are made to operate, to produce the effect. Engines differently constructed, and particularly those upon a larger scale than ordinary pumps, are generally termed Hydraulic Machines, which we have already treated of under that head. The ordinary definition of pump is, "a machine in which water is raised by the pressure of the atmosphere," which accords with the prevalent but erroneous notion, that the atmosphere does of itself raise water to a height of thirty feet; notwithstanding it is known to those who have considered the subject, that it does not, in fact, contribute in the slightest degree to raise it at all; and that the same force is requisite to raise a pound of water a given height, as to raise a pound of lead, or of any other substance, through the same space. Of the evident truth of this fact, the reader, if a novice, will be satisfied upon reading our article on hydraulics or hydrodynamics, and by attending to the following description of a