Nevertheless the instrument now about to be described, depending upon similar principles, but scientifically constructed and graduated, is a very useful and valuable substitute for the mercurial barometer. It consists of a glass tube, varying, according to the purposes for which the instrument is required, from six to twenty-four inches in length. The upper end is closed, and formed into a bulb; the lower is turned up, formed into a cistern, and open at top, through a pipette, or cone. A plug, moveable by a catch from below, can be made to close this opening, so as to render the instrument portable. [Illustration: Fig. 32.] The upper portion of the tube is filled with air; the lower portion, and part of the cistern, with sulphuric acid, coloured so as to render it plainly visible. Formerly, hydrogen and oil were used. It was found, however, that, by the process known to chemists as _osmosis_, this light gas in time partially escaped, and the remainder became mixed with air, the consequence being that the graduations were no longer correct. They are more durable as at present constructed. The liquid rises and falls in the tube with the variations of atmospheric pressure and temperature acting together. If the pressure were constant, the confined air would expand and contract for temperature only, and the instrument would act as a thermometer. In fact, the instrument is regarded as such in the manufacture; and the thermometric scales are ascertained and engraved on the scale. A good mercurial thermometer is also mounted on the same frame. If, therefore, at any time the mercurial and the air thermometers do not read alike, it must evidently be due to the atmospheric pressure acting upon the air in the tube; and it is further evident that, under these circumstances, the position of the top of the liquid may be marked to represent the barometric pressure at the time. In this manner a scale of pressure is ascertained by comparison with a standard barometer, extending generally from 27 to 31 inches. When made correctly, these instruments agree well with the mercurial barometer for a number of years, and their subsequent adjustment is not a matter of much expense. For use at sea, the liquid column is contracted at the bend. The sympiesometer is very sensitive, and feels the alterations in the atmospheric pressure sooner than the ordinary marine barometer. The scale is usually on silvered brass, mounted on a mahogany or rosewood frame, protected in front by plate glass. It is generally furnished with a revolving register, to record the observation, in order that it may be known whether the pressure has increased or decreased in the interval of observation. Small pocket sympiesometers are sometimes fitted with ivory scales, and protected by a neat velvet-lined pasteboard or morocco case.