In the previous observations, general reference has been made to mercurial barometers of the ordinary kind; but, excepting the construction of the instruments themselves, those observations apply to all barometers, wheel--aneroid--or metallic--and likewise, of course, to the sympiesometer, which is a modified barometer. But as these four last-mentioned instruments are scarcely so familiar as the simplest form of barometer, it may be useful to add a few words about each of them.
The WHEEL barometer has a syphon tube, partly filled with mercury, on which, at the short or open end of the tube, a float moves, to which a line is attached that moves a wheel, carrying an index.[31]
ANEROID barometers, if often compared with good mercurial columns, are similar in their indications, and valuable; but it must be remembered that they are not independent instruments; that they are set originally by a barometer,[32] require adjustment occasionally, and may deteriorate in time, though slowly.
The aneroid is quick in showing the variation of atmospheric pressure, and to the navigator who knows the difficulty, at times, of using barometers, this instrument is a great boon, for it can be placed anywhere, quite out of harm's way, and is not affected by the ship's motion, although faithfully giving indication of increased or diminished pressure of air.[33] In ascending or descending elevations, the hand of the aneroid may be seen to move (like the hand of a watch), showing the height above the level of the sea, or the difference of level between places of comparison.[34]
The principle on which it is constructed may be explained in a few words, without going into a scientific or too minute detail of its various parts. The weight of a column of air, which in a common barometer acts on the mercury, in the aneroid presses on a small circular metal box, from which nearly all air is extracted; and to this box is connected, by nice mechanical arrangement, the hand visible over the face of the instrument. When the atmospheric pressure is lessened on the vacuum box, a spring acting on levers, turns the hand to the left, and when the pressure increases, the spring is affected differently, the hand being turned to the right. It acts in any position, but as it _often varies several hundredths with such a change_, it should be held uniformly, while read off.