riginates in the corrugated elastic box
_a_, the surface of which is depressed or elevated as the weight of the
atmosphere is increased or diminished, and this motion is communicated
through the levers to the axis of [v.03 p.0421] the hand at _h_. The spiral
spring on which the lever rests at _d_ is intended to compensate for the
effects of alterations of temperature. The actual movement at the centre of
the exhausted box, whence the indications emanate, is very slight, but by
the action of the levers is multiplied 657 times at the point of the hand,
so that a movement of the 220th part of an inch in the box carries the
point of the hand through three inches on the dial. The effect of this
combination is to multiply the smallest degrees of atmospheric pressure, so
as to render them sensible on the index. Vidie's instrument has been
improved by Vaudet and Hulot. Eugene Bourdon's aneroid depends on the same
principle. The aneroid requires, however, to be repeatedly compared with a
mercurial barometer, being liable to changes from the elasticity of the
metal chamber changing, or from changes in the system of levers which work
the pointer. Though aneroids are constructed showing great accuracy in
their indications, yet none can lay any claim to the exactness of mercurial
barometers. The mechanism is liable to get fouled and otherwise go out of
order, so that they may change 0.300 in. in a few weeks, or even indicate
pressure so inaccurately and so irregularly that no confidence can be
placed in them for even a few days, if the means of comparing them with a
mercurial barometer be not at hand.

[Sidenote: Barographs.]

The mercurial barometer can be made self-registering by concentrating the
rays from a source of light by a lens, so that they strike the top of the
mercurial column, and having a sheet of sensitized paper attached to a
frame and placed behind a screen, with a narrow vertical slit in the line
of the rays. The mercury being opaque throws a part of the paper in the
shade, while above the mercury the rays from the lamp pass unobstructed to
the paper. The paper being carried steadily round on a drum at a given rate
per hour, the height of the column of mercury is photographed continuously
on the paper. From the photograph the height of the barometer at any
instant may be taken. The principle of the aneroid barometer has been
applied to the construction of barographs. The lever attached to the
collapsible chamber te