MANOMETER (from two Greek words—pawls, thin or rare, and pirpos, a measure) is the name given to instruments which measure the rarity of the atmosphere or other gas. As however the rarity of a gas is proportional to its elastic force, so long as its temperature and chemical composition remain unchanged, such instruments as measure the elastic force of gases are also, with this restriction, properly termed manometers, and accordingly it is to the latter instruments that the term is most frequently applied, both in this country and upon the Continent.
The " statical barometer " of Robert Boyle was a manometer of the simplest kind, consisting of an exhausted glass globe suspended from one extremity of a delicate balance, and counterpoised by a metallic weight at the other extremity, the adjustment being made when the atmosphere was in its mean state of density. Any subsequent variation in the specific gravity of the air would, by a known law of hydrostatics, destroy the equilibrium, and the motion of the globe would indicate whether the variation had inclined towards an increase or diminution of density, as in the former case it would ascend, in the latter it would descend.
Captain Phipps, in his north-polar voyage, and Colonel Roy, in order to correct his barometric observations, employed manometers, which gave the elastic tension of the atmosphere. They consisted of glass tubes similar in form to thermometer-tubes, and of various sizes. Those of Colonel Roy were from 4 to 8 feet in length, with bores from nth to it.th of an inch in diameter. The bulb and part of the tube being filled with air of known tension, and the remainder of the tube being partially occupied by a small column of mercury sufficient to cut off the communication between the internal and external air ; any variation in the elastic tension of the latter, arising from change of weight, would be accurately measured by the ascent or descent of the mercurial column ; for whenever the tension of the atmosphere exceeded that of the contained air, the column would move towards the bulb, and the contrary. But if the change in the tension of the atmosphere were partly attributable to a change of temperature, then the motion of the column would merely measure the difference of the variations in the tension of the internal and external air, because the tension of both would be equally affected by the change of temperature. The bulb was pear-shaped, so that the " point being occasionally opened, dry or moist air could be readily admitted, and the bulb sealed again without any sensible alteration in its capacity." (' Phil. Trans., vol. lxvii.,
p. 6S9.) The manometers of Varignon and Wolf were similar to the preceding.
A more convenient instrument, and one of more general use, con sists of a siphon-barometer, the basin of which is enclosed air-tight in a globular or other conveniently shaped vessel, furnished with a number of cocks, by means of which and the pneumatic pump the contained gas may be removed, and other gases successively substituted in its place. If equal parts by weight of different gases be thus successively introduced, they will not bo affected by any change which may take place in the surrounding atmosphere, except in so far as such change may affect their temperature ; so that, providing the temperature remain constant, the relative tensions of these gases will be accurately measured by the weight of the mercurial column suspended in the longer arm of the barometer, above the level of the mercury in the basin ; care being had to allow for any variation in the capacity of the receiver, arising from alteration in the level of the mercury in the basin, and likewise for the small tension always indicated by the barometer immediately previous to the introduction of a fresh gas, arising from the impossibility of forming a perfect vacuum.
If an approximate vacuum be formed in the receiver enveloping the basin of the barometer, and a small quantity of any liquid be then introduced, it will be immediately converted into vapour, and the elastic tension of this vapour will be measured in precisely the same way as that of permanent gases. The receiver is sometimes of suffi cient size to contain animals and plants, the effect of which in increasing or diminishing the tension of the enclosed gas is then measured by the rise or fall of the mercury. If this manometer be transported from one place to another, where the temperature is the same, but the force of gravity different, this variation in the force of gravity will be mani fested by a corresponding variation in the length of the mercurial column ; that is, if the gravity increase, the tension of the enclosed gas will be counterbalanced by a shorter column of mercury, and the contrary; but as this method of measuring the variations in the force of gravity is not susceptible of that accuracy which is attained by the employment of the pendulum, it is rarely if ever resorted to.