[1] The use of the Barometer on Surveys and Reconnoisances. By R. S. Williamson. New York: D. Van Nostrand. London: Trubner & Co.
CHAPTER III.
ANEROID BAROMETERS:
THEIR CONSTRUCTION.
The general principle of construction of all aneroids is the same. A box with flexible sides, hermetically sealed, the air having been first exhausted, changes its form as the pressure of the atmosphere varies.
The chief differences in the various kinds lie in the mechanical devices, by which the motions of the box are rendered apparent to the eye, and also measured in such a manner as to allow the corresponding pressures to be expressed in inches of mercury.
Fig. 1.
The aneroid was invented about the beginning of this century, but was first made of a serviceable form by Vidi, in 1848. It is substantially the form most used to-day. The vacuum box is a thin low cylinder, and the motion of the thin flexible head of the cylinder is conveyed by suitable mechanism to the index hand. Vidi’s aneroid is shown in [Fig. 1]. D is the vacuum box, supporting the upright pillar M upon its center. As M rises or falls, a corresponding motion is given to the plate C. A counter-pressure is afforded by the spiral spring S. The motion of C is conveyed by the links 1 and 2 to a little rocker shaft, shown in the [figure]. An arm, 3, attached to this shaft is connected by a minute chain with the shaft which carries the index pointer. It is kept wound to the proper tension about this shaft by a fine spiral hair spring.
Fig. 2.
