somewhat resembles in appearance a carriage clock or a ship’s chronometer; internally, it consists of a small air-tight cylindrical box, formed of thin corrugated copper plates, and partially exhausted of air, the sides of which yield to the pressure of the atmosphere; the effect being regulated by a spring, multiplied by a system of levers, and ultimately recorded by the index on a graduated dial. Compensation for changes of temperature are self-effected, with almost perfect accuracy, by the elastic force of the spring being so adjusted to that of the air in the cylinder, that the loss of force in the one and the increased expansive force of the other shall, independently of changes of atmospheric pressure, preserve the lever in equilibrio.
[109] An instrument founded on the same principle, and of nearly similar construction was described by M. Conté, in 1798, in the ‘Bull. des Sci. Nat.,’ t. i, No. xiii, p. 106.
The indications of the aneroid barometer closely correspond to those of the mercurial barometer at ordinary ranges; the differences never exceeding ·01 of an inch. It is so extremely sensitive that an ascent or descent of only a few feet is distinctly indicated by it; whilst its portability adapts it for service in situations for which an ordinary barometer is unfitted. On the other hand, it is liable to move by jerks, and the elasticity of the spring, and consequently the zero-point of the scale, has been found to be sometimes affected by time and a rough journey. On this account it is necessary to compare it occasionally with some standard mercurial barometer, to determine its amount or rate of variation, if any.
Barometer, Phi′al. This amusing philosophic toy is made by cutting off the rim and part of the neck of a common glass-phial with a file. The phial is then nearly filled with water, either pure or tinged blue or red; and the finger being placed on its mouth, it is inverted, and suspended in a vertical position by means of a piece of twine or wire, when the finger is withdrawn. (See engr.) In dry weather the under surface of the water remains level with the neck of the bottle, or even concave; in damp weather, on the contrary, a drop appears at the mouth and continues enlarging until it falls, and is then followed by another in the same way.
Barometer, Fitzroy. This, which is a very cheap instrument, is made on the syphon principle, but the cistern is formed by the lower limit, which is blown into a bulb.
Barometer, Port′able. The most accurate are those of Gay-Lussac and Bunten, and after them the aneroid. They should be set on universal joints, and well balanced. The common instrument made with a box and leather cistern seldom continues long correct.
Barometer, Wheel. The common form of the instrument having a dial-face and hands.