METEOROLOGICAL INSTRUMENTS

145

spacing on the fixed scale is '05 of an inch, and this is divided into five
equal parts by means of the enlarged divisions numbered 1, 2, 3, 4, 5
on the vernier; these parts, however, are really -01, -02, -03, -04, -05,
the -0 being left out for convenience. Run the eye up the vernier until
one of its divisions is exactly coincident with any division of the fixed
scale. It is the second one above 3 which is '034 to add to 29*650 making
the reading 29'684. The short divisions between the numbered ones
on the vernier are in succession -002, -004, -006, -008 of an inch.

Fig. 13.—Millibar Scale and Vernier. The Reading is 1012-7 mb.
Figure 13 shows part of a millibar scale. AB is the fixed scale
from 990 mb., to 1060 mb. CD is the sliding vernier, D is the top of
the mercury column. The reading is 1012-7 millibars. The millibars
are read from the AB scale and the decimal from the GD scale.
Instrumental Corrections for Latitude, temperature and height above
sea level are necessary in order to reduce the observation to a common
standard for the purpose of comparison with the readings got from
other barometers.
The standard Latitude is 45°, this correction is necessary because the
force of gravity is greater at the poles than at the equator, due to tJie
polar diameter of the earth being shorten than the equatorial