METEOROLOGICAL INSTRUMENTS 3

by means of clock work, making a complete revolution in seven da
The vertical lines printed on the barograph chart represent time, a
the horizontal lines either inches or millibars. The pen moves up a
down in response to changes of pressure, and the revolving dri
imparts a horizontal motion to the paper which slips round under the p«

14.—A Barograph

The chief advantage lies in the fact that a continuous record c
changes in pressure is presented in a graphic form, by means of whic
the history of the barometrical changes may be read at a glance, eve
slight fluctuations of pressure due to passing squalls being recorded.
Barograph readings are subject to the same corrections as th<
aneroid, in addition to regulating its time at sea. The drum rotates a
a uniform rate, while the ship time changes as she sails East or West
the drum should therefore be set to G.M.T. and the ship's Longitude
noted on the barograph chart.
THE THERMOMETER.
Construction and Principle.—It consists of a glass tube of very
small bore, having a bulb attached to one end, sealetl at the other, and
partially filled with mercury, or with spirits of wine if the instrument
is required for very low temperatures.
Bodies expand with heat and contract with cold, and as mercury
expands to a greater degree than glass we find the thin thread of mercury
rising and falling in the tube as the temperature increases and decreases
respectively*
The two fixed points of the scales used in making thermometers
are 'the freezing point and boiling point of distilled water, when the
barometrical pressure is 30-0 inch