Barometer, An′eroid

An instrument invented, or at least perfected, by M. Vidi, of Paris, in which the pre ure of the atmosphere is measured without the employment of a fluid, as in the ordinary barometer. [109] Externally, it 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 pre ure 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 lo of force in the one and the increased expansive force of the other shall, independently of changes of atmospheric pre ure, 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. IMG:596405606043057898_i268-1.png: IMG:596405606043057898_i268-2.png: 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 nece ary to compare it occasionally with some standard mercurial barometer, to determine its amount or rate of variation, if any. IMG:596405606043057898_i268-3.png: