to have recourse to artificial means
of changing the pressure of the atmosphere on the surface of the mercury
in the cistern.

The barometers intended to be tested are placed, together with a
Standard, in an air-tight chamber, to which an air pump is applied, so
that, by partially exhausting the air, the Standard can be made to read
much lower than the lowest pressure to which marine barometers are
likely to be exposed; and by compressing the air it can be made to read
higher than the mercury ever stands at the level of the sea. The tube of
the Standard is contracted similarly to that of the marine barometer,
but a provision is made for adjusting the mercury in its cistern to the
zero point. Glass windows are inserted in the upper part of the iron
air-chamber, through which the scales of the barometers may be seen; but
as the verniers cannot be moved in the usual way from outside the
chamber, a provision is made for reading the height of the mercury
independent of the verniers attached to the scales of the respective
barometers. At a distance of some five or six feet from the air-tight
chamber a vertical scale is fixed. The divisions on this scale
correspond exactly with those on the tube of the Standard barometer. A
vernier and telescope are made to slide on the scale by means of a rack
and pinion. The telescope has two horizontal wires, one fixed, and the
other moveable by a micrometer, screw so that the difference between
the height of the column of mercury and the nearest division on the
scale of the Standard, and also of all the other barometers placed by
the side of it for comparison, can be measured either with the vertical
scale and vernier or the micrometer wire. The means are thus possessed
of testing barometers for index error in any part of the scale, through
the whole range of atmospheric pressure to which they are likely to be
exposed, and the usual practice is to test them at every half inch from
27.5 to 31 inches.

In this way barometers of various other descriptions have been tested,
and their errors found to be so large that some barometers read half an
inch and upwards too high, while others read as much too low. In some
cases those which were correct in one part of the scale were found to be
from half an inch to an inch wrong in other parts. These barometers were
of the old and ordinary construction. In some the mercury would not
descend lower than about 29 inches, owing to a fault very common in t