o begin to descend.

In reading off from a barometer, it should hang freely, not inclined by
holding, or even by touch.

Sometimes, though rarely, at sea the mercury seems _stopped_. If so,
take down the instrument (after _sloping_), reverse it, tap the tube
gently while the cistern end is upwards, and then replace as before.

* * * * *

TESTING BAROMETERS, HYDROMETERS, AND THERMOMETERS.

In the year 1853 a conference of maritime nations was held at Brussels,
on the subject of meteorology at sea. The report of this conference was
laid before Parliament, and the result was a vote of money for the
purchase of instruments and the discussion of observations, under the
superintendence of the Board of Trade. Arrangements were then made, in
accordance with the views of the Royal Society and the British
Association for the Advancement of Science, for the supply of
instruments properly tested.

In the barometers now in general use by meteorologists on land, the
diameters of the tubes are nearly equal throughout their whole length,
and a provision is made for adjusting the mercury in the cistern to the
zero point, previous to reading the height of the top of the column. The
object of the latter arrangement, it is well known, is to avoid the
necessity of applying a correction to the readings for the difference of
capacity between the cistern and the tube. At sea, barometers of this
construction cannot be used. Part of the tube of the marine barometer
must be very much contracted to prevent "pumping," and the motion of the
ship would render it impracticable to adjust the mercury in the cistern
to the zero point. In the barometer usually employed on shore, the index
error is the same throughout the whole range of scale readings, if the
instrument be properly made; but in nearly all the barometers which have
till recently been employed at sea, the index correction varies through
the range of scale readings, in proportion to the difference of capacity
between the cistern and the tube. To find the index correction for a
land barometer, comparison with a Standard at any part of the scale at
which the mercury may happen to be, is generally considered sufficient.
To test the marine barometer is a work of much more time, since it is
necessary to find the correction for scale readings at about each half
inch throughout the range of atmospheric pressure to which it may be
exposed; and it becomes necessary