METEOROLOGICAL INSTRUMENTS 143
barometer. When the pump is at rest the atmospheric pressure in the
chamber is equal to the pressure on the surface of the water in the well.
When the pump is worked, a partial vacuum is created in the chamber,
thus diminishing the pressure, so that the relatively greater pressure
exerted on the surface of the water m the well raises th& water into
the pump chamber.
In like manner, the pressure having been removed from tne inside
of the tube of the barometer by expelling the air, the pressure of the
atmosphere on the surface of the mercury in the cistern forces the
mercury up the tube, until the weight of the column is equal to that
pressure.
A water barometer would require a tube about 33 feet in length, but
mercury, being 13J times heavier than water, only requires a tube
about 32 inches in length.
Pumping is the name given to the rising and falling of the mercury
caused by the heaving of the ship at sea, or the rocking of the instrument.
Pumping is reduced by contracting the bore of the tube and also by
slinging the barometer in gimbals.
Certain precautions have to be observed when taking down a
barometer and packing it for transport to protect it from damage.
Unship it from the bracket and handle it carefully. The instrument
should be brought into a horizontal position very gradually in order to
allow the mercury to flow gently to the top of the tube. Put the baro-
meter in its box and use soft packing, avoid jars and concussion, and
carry the box horizontally with cistern end tilted up slightly. If
sending by post or rail, put the label on the end next to the cistern,
and mark it boldly "scientific instruments—glass—fragile—keep fiat
or this end up."
If a mercurial barometer were brought quickly from an upright to
a horizontal position the weight of the mercury would probably break
the glass as there is no air at the top of the tube to act as a cushion.
A Millibar Js the thousandth part of a "Bar/' which is the unit
adopted by meteorologists to express the average pressure of the
atmosphere at sea level and is approximately 14J Ibs. per square
inch—1000 millibars=29*5 inches of mercury.
The Vernier is a sliding scale by which more accurate readings of a
fixed scale may be obtained. The principle of its construction is, that
a giv§n length of vernier, equal to a certain number of divisions of the
fixed scale, is divided into one more or one less than that number of
divisions. The incJi vernier of the marine barometer i* usually equal