MANOMETER (Gr. µavos, thin or loose; µerpov, a measure)  , an instrument for measuring the pressures exerted by gases or vapours . An alternative name is pressure gauge, but this term may conveniently be restricted to manometers used in connexion with steam-boilers, &c . The principle of hydro-statics suggest the most common forms . Suppose we have a U tube (fig . 1), containing a liquid: if the pressures on the surfaces of the liquid be equal, then the surfaces will be at the same height . If, on the other hand, the pressure in one limb be greater than the pressure in the other, the surfaces will be at 2 Manoeuvres incidentally afford an excellent opportunity of testing new patterns of equipment, transport or other materiel under conditions approximating to those of active service . different heights. the difference being directly proportional to the difference of pressures. and inversely as the specific gravity of the liquid used . Two forms are in use: (I) the " open-tube," in which the pressure in one limb is equal to the atmospheric pressure, and (2) the' closed-tube," in which the experimental pressure is balanced against the liquid column and the air compressed into the upper part of a closed limb of the tube . In the " open tube " form (fig . I) the pressure on the surface a is equal to the pressure on the surface at b (one atmos1,here) plus the hydrostatic pressure exerted by the liquid column of height a b . The liquid commonly used is mercury . If a scale be placed behind the limbs of the tube, so that the difference a b can be directly determined, then the pressure in a is at once expressible as P + a b in millimetres or inches of mercury, where P is the atmospheric pressure, known from an ordinary barometric observation .

In the " closed tube " form (fig . 2) the calculation is not so

simple, for the variation of pressure on the mercury surface in the closed limb has to be taken into account . Suppose the length of the air column in the closed limb be h when the mercury is at the same height in both tubes . Applying the experimental pressure to the open end, if this be greater than atmospheric pressure the mercury column will rise and the air column diminish in the closed limb . I.et the length of the air column be h', then its pressure is h/h' atmospheres . The difference in height of the mercury columns in the two limbs is 2 (h-h'), and the pressure in the open limb is obviously equal to that of a column of mercury of length 2(h-h'), plus h/h' atmospheres . These instruments are equally serviceable for deter-mining pre. sores less than one atmosphere . In laboratory practice, e.g. when it is required to determine the degree of exhaust of a water pump, a common form consists of a vertical glass tube having its lower end immersed in a basin of mercury, and its upper end connected by means of an intermediate vessel to the exhaust . The mercury rises it the tube, and the difference between the barometric height and the length of the mercury column gives the pressure attained .