142

NICHOLLS's SEAMANSHIP AND NAUTICAL KNOWLEDGE

4

THE MARINE BAROMETER.

Tbrrtcelltan vacuum-
Top of column of mercury-

Graduated Scats

\ to which The vernier

is attached —

npart of Tube—*•

Tube__^

bore)

Screw with —^
milled head for
actuating uernttr

Attached
Thermometer^

Bulkhead

The barometer is an instrument for measuring variations in the weight
or pressure of the atmosphere.
The most important part of it is a glass tube containing mercury. This
tube is airtight at the top and open at the bottom. It is fitted in a case with
its open end immersed in a cistern also containing mercury. The pressure oi
the atmosphere on the surface of the mercury in the cistern maintains the
column of mercury in the tube at a height which corresponds to that pressure,
Readings are obtained by means of a graduated scale and vernier.
The left-hand drawing represents the glass tube taken out of its case so
that the different parts of it can be seen. The other one shows the complete
instrument suspended from a bulkhead by means of an arm and bracket.