Previous to making an observation with the barometer the instrument
should be slightly tapped to free the mercury from any adhesion to the
glass; any violent oscillation should, however, be carefully avoided.
The vernier should then be adjusted to the upper surface of the mercury
in the tube; for this purpose its back and front edges should be made to
coincide, that is, the eye should be placed in exactly the same plane
which passes through the edges; they should then be brought carefully
down until they form a tangent with the curve produced by the convex
surface of the mercury and the light is _just_ excluded from between
them and the point of contact. It is desirable in making this adjustment
that the eye should be assisted by a magnifying-glass. The reading of
the scale should then be taken and entered in the column appropriated to
it in the proper form. If the instrument have no tubular or double-edged
index, the eye should be placed carefully at the level of the upper
surface of the mercury and the index of the vernier brought gently down
to the same level so as apparently just to touch the surface, great care
being taken that the eye index and surface of the mercury are all in the
same plane.

Each observation of the barometer should be accompanied by an
observation of the direction of the wind, which should be noted in the
usual manner in which it is observed at sea. In connexion with the
_direction_ the _force_ of the wind should be recorded in accordance
with the following scale, contrived by Admiral Sir Francis Beaufort:--

0. Calm
1. Light air or just sufficient to give steerage way.
2. Light breeze { or that in which a well- } 1 to 2 knots.
3. Gentle breeze { conditioned man of war, } 3 to 4 knots.