146 NICHOLLS'S SEAMANSHIP AND NAUTICAL KNOWLEDGE
The standard height is sea level, the correction being about one-
tenth of an inch for every hundred feet, and is due to the atmospheric
pressure decreasing with the height.
The standard temperature is 32° Fahr ; this correction is necessary
because the column of mercury lengthens and shortens with heat and cold.
The value of these several corrections may be found in the Barometer
Manual.
Errors of Capillarity and Capacity.—Capillarity is due to the surface
tension of the mercury and the frictional resistance between the mercury
and the glass tube. It depresses the level of the mercury.
Capacity is due to the changes in the level of the mercury in the
cistern as it rises and falls in the tube.
The scale of marine barometers is graduated to obviate the necessity
of applying these two corrections, the mercury inches being shorter
than lineal inches.
An Aneroid Barometer consists of a circular metallic chamber
partially exhausted of air and hermetically sealed. Variations in the
pressure of the atmosphere produce changes in the volume of the
chamber, and by an arrangement of levers and springs the motion thus
imparted to the chamber is communicated to a hand which indicates
the prevailing pressure.
It is not so reliable as the mercurial barometer, with which it should
be frequently compared, nor is it possible to find with sufficient accuracy
for scientific purposes the index error of the aneroid.
Aneroid readings require correction for height above sea level and
index error, but not for tetnperature or latitude.
An aneroid is popular on board ship because it is very handy and
also more sensitive to small changes of pressure than the mercury
barometer. Space often being an important consideration, it can be
hung in positions which would be impossible for a mercury barometer.
Having no vernier to adjust, readings are quickly and easily obtained.
Being used principally to indicate changes of pressure as an aid to
forecasting the weather, it is not customary to apply any corrections to
its reading.
A Barograph is a self-recording aneroid. It consists of a series of
vacuum metal boxes with elastic lids. The volume of the boxes changes
with every variation of pressure in the same way as in the aneroid
barometer. The expansion and contraction thus caused are communi-
cated through a lever to a pen, which marks an ink trace on a paper.
This papex is wound round a drum, which is rotated^on a vertical axi*