Clark Ross. These instruments were similar to the ordinary portable
barometers, and differed from them only in the mode of their suspension
and the necessary contraction of the tubes to prevent oscillation from
the motion of the ship. The barometer on shipboard should be suspended
on a gimbal frame, which ought not to swing too freely, but rather so as
to deaden oscillations by some degree of friction. To the upper portion
of the tube in this construction of instrument light is alike accessible
either in front or behind, and the vernier is furnished with a back and
front edge, both being in precisely the same plane, nearly embracing the
tube, and sliding up and down it by the motion of rack-work; by the
graduation of the scale and vernier the altitude of the mercury can be
read off to ·002 inch.

When the barometer is placed in the ship, its position should be as near
midships as possible, out of the reach of sunshine, but in a good light
for reading, and in a situation in which it will be but little liable to
sudden gusts of wind and changes of temperature. Great care should be
taken to ascertain the exact height of its cistern above the water-line,
and in order to facilitate night observations every possible arrangement
should be made for placing behind it a light screened by white paper.

_Observations._--The first thing to be done is the reading off and
recording the temperature indicated by the thermometer that in this
construction of instrument dips into the mercury in the cistern. Sir
John Herschel has suggested that "the bulb of the thermometer should be
so situated as to afford the best chance of its indicating the exact
mean of the whole barometric column, that is to say, fifteen inches
above the cistern enclosed within the case of the barometer, nearly in
contact with its tube, and with a stem so long as to be read off at the
upper level."