METEOROLOGICAL INSTRUMENTS 151
continuous record of temperature. The thermometer consists of a
slightly curved metal tube filled with spirit, on<3 end of the tube being
fixed rigidly to the instrument, and the other end attached to the
system of levers which actuates the recording pen.
The great advantage of the thermograph, especially if studied in
connection with a barograph, is to demonstrate the close relationship
which exists between the fluctuations of temperature and pressure.
Thermographs must be exposed out of deors and consequently the
bearings require to be frequently oiled and examined. The instrument
takes an appreciable time to alter in temperature, so it is apt to be
sluggish when the changes of temperature are rapid.
MASON'S HYGROMETER
It consists of two ordinary thermometers placed side by side. One,
called the wet bulb, has a piece of cambric tied to the bulb, and a few
strands of cotton wick fastened to the cambric with their lower ends
dipping into a cup of water. The cambric is thus kept moist. The
thermometers should be mounted in an open screen through which the
air passes freely, and away from the effects of heated currents of aii
from cabins, etc.
Care should be taken to keep the, cambric and wick clean, and the
cup replenished with a supply of fresh water. Should the cambric and
wick get wet with salt spray, they should be cleaned in fresh water, and
care should be taken that no water is adhering to the dry bulb when
the readings are noted.
The hygrometer measures the humidity of the air. When the air
is dry, evaporation takes place and the water dries off the cambric,
thus reducing the temperature of the wet bulb thermometer; when
the air is moist there is less evaporation so that the difference between
the wet and dry bulb readings is correspondingly less.
Consequently, in damp weather or when the air is saturated, there is
little or no difference in the readings, but in dry weather at sea the
wet bulb may be as much as 10° lower than the dry bulb (Fig. 17).
A RAIN GAUGE
This instrument is not usually included in the weather recording
apparatus supplied to ships, but if it were carried it would be hoisted
to a suitable height above the deck where it would be free from the
sheltering effects of deckhouses, etc., also from sprays