BAROMETER (Gr. barns, weight; metros, a measure), an instrument for measuring the weight or pressure of the atmosphere. The term is generally understood to refer to one in which the measure is the height of a column of liquid sustained by atmospheric pressure. The fundamental principle of the construction of the B. is best shown in the experiment which led Torricelli to the first discovery of the pressure of the air. A glass tube, about 33 in. in length, open at one end, is completely filled with mercury, and, being firmly closed by the thumb, is inverted and placed vertically in a cup containing mercury. When the thumb is removed, the mercury sinks in the tube till it stands, generally, about 30 in. above the level of the mercury in the cup, leaving in the upper part a space free of air, which receives the name cf the Torrieellian vacuum (fig. 1). Phe mercury within the tube being thus removed from the pressure of the air, while that in the cup is exposed to it, the column falls, till the pressure at the section of the whole, in the same plane as the surface of the mercury in the cup, is the same within and without the tube. A similar experiment is seen when, in a U-shaped tube, having one branch much wider than the other, a column of mercury in the narrow branch balances a column of water nearly 14 times as high in the other. In the Torricellian experiment, we have the air and the space occupied by it taking the place of the.wide water branch of the U-shaped tube, and the glass tube and mercury forming the narrow branch, as before; the narrow branch, howeyer, in this case being closed above, to pre vent the air from filling, as it were, both branches. In both cases, the heights of the columns are inversely as the specific gravities of the liquids of which they consist; and, as air is about 10,000 times lighter than mercury, we should have the aerial column 10,000 times 30 in. high. It will be found, under ATMOSPHERE, that from the air lessen ing in density as it ascends, the height is considerably greater. Any changes that take place in the height or density of the aerial column will be met by corresponding changes in the height of the mercurial column, so that as the latter rises or falls, the former increases or diminishes. We have, then, in this simple tube, an infallible index of the varying amount of atmospheric pressure, and, in fact, a perfect barometer. The changes, how ever, are indicated on a scale at least 10,000 times diminished, so that the variations in the tube show very considerable changes in the weight of the atmosphere. If water be used instead of mercury, the water column would be 14, or, more correctly, 13.0 times as high as the mercurial column, or about 34 ft. ; and the scale on which the changes take place would be correspondingly magnified, so that a water B. should be much more delicate than a mercurial one. Water is, however, exposed to this serious objection, that
its vapor rises into the empty space above, and causes by its elasticity a depression of the column, the depressions being different for different temperatures. At zero, Fahren heit, for instance, the depression thus arising would be 4- an inch, and at 77', more than 1 foot. It would be doubtful, likewise, at the time of any observation, whether the space referred to was filled with vapor of the elasticity corresponding to the observed external temperature or not, so that the necessary correction could not with certainty be made. The vapor of mercury, on the other hand, at 77' F.—a temperature considerably above the average—produces iu the B. a depression of only of an inch, an amount practically inappreciable. After 200 years of experience and invention, we have yet no better index of the pressure of the atmosphere than the simple mercurial column of Tor ricelli, and in all exact observations it is taken as the only reliable standard.
Simple as the B. is, its construction demands considerable care and experience. It is of the first importance that the mercury to be used is chemically pure, otherwise its fluidity is impaired, and the inside of the tube becomes coated with impurities in such a way as to render correct observation impossible. Mercury, as usually sold, is not pure; and before being employed for barometers, must be shaken well with highly dilute but pure nitric acid, to remove extraneous metals and oxides. The same object is effected more thoroughly by keeping it several weeks in contact with the dilute acid, stirring ,every now and then. After either process, the metal must be thoroughly washed with distilled water, and dried. In filling the tube, it is essentially necessary to get the col umn free from air and moisture. To effect this, the mercury, after filling, is boiled in the tuba, so that air and moisture may be expelled, partly by the heat, and partly by the vapor of the mercury. This process demands great experience and skill, but the same end may be more easily and as effectually attained by the mercury, in the first instance, in an atmosphere of carbonic acid, and then pouring it into the previously heated tube by a filler reaching to the bottom of it. Such care is only expended on the best instruments; ordinary weather-glasses, not needing to be quite accurate, arc more simply filled. Notwithstanding all these precautions, minute bubbles of air manage to keep secreted, and creep up in the course of time into the Torricellian vacuum. To obviate this risk of error, an air-trap is recommended by which any air that may acci dentally find its way into the tube is arrested in its ascent to the. top, and the instrument sustains no damage from the accident.