MA.NOM'ETER (ante). The various forms of manometer may be classified under three heads: 1, the open-air manometer, on the principle of the barometer; 2, the con 'fined-air manometer, on the principle of Mariotte's instrument (q.v.); and 3, the metallic spring manometer. A shnple open-air manometer consists of a glass tube, open at both •ends, placed upright in a strong bottle of glass or iron, the bottom of which contains mercury. The tube passes through a tight packing box in the neck. In the upper part of the bottle there is an orifice which admits compressed air, acted upon by steam or -vapor, whose tension it is desired to measure. But this form cannot be used for high -pressures. The multiple-branch manometer is a modification of the simple open instru ment, and is constructed by bending a long tube,,open at both ends, in a series of V shaped flexures of from 20 to 40 in. in heat, the number of flexures depending upon the pressure the instrument is liable to be subjected to. Columns-of mercury, of equal -height, being placed in the lower halves of the V-shaped legs, will indicate the pressure excited at one end of the tube, by the sum'of the excess of height of the mercurial col umns in alternate legs, or by multiplying the excess of height in one leg by the number of le„gs containing such excess. The system is fastened to a board or metallic plate, which at one side, near the last branch, is furnished with a graduated scale. The com
pressed-air manometer is simply a strong V-shaped tube closed at one end, while at the other is attached the pipe comtnunicating with the gas or vapor whose tension it is desired to measure. A portion of the flexure of the V contains mercury, and the space between it and the closed end is filled with common air. Now, according to Boyle's or Mariotte's law, a pressure exerted on the column of mercury sufficient to force the air into half the space it occupies at the nortual atmospheric pressure, must become doubled, -or 15 lbs. to the square inch must be added. Again, to compress the air into half the remaining space, ia lbs., or double the pressure required for the reduction to the first half, must be added, making in all a pressure of four atmospheres for the reduction to one-fourth the original volume. It is evident, therefore, that a graduated scale, to exhibit the degrees of pressure, must have its spaces decrease from below upwards. The graduation is accotnplished by means of an open-air multiple manometer. The metallic -spring manometer consists of an index traversing a graduated arc, and having applied to a spring connected with it—which may be in the form of a spiral—a piston actuated by the force of the gas or vapor in the boiler or steam-chamber.