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preface to fifteenth edition

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GENERAL INFORMATION, CONVERSION TABLES, AND MATHEMATICS 2.71<br />

This correction converts the reading in<strong>to</strong> the value of 0C. The brass scale table is applicable <strong>to</strong> the<br />

Fortin barometer. See Tables 2.10 latitude-gravity correction, and 2.11 altitude-gravity correction,<br />

<strong>to</strong> compensate for differences in gravity, which would affect the height of the mercury column by<br />

variation in mass. If local gravity is unknown, an approximate correction may be made from the<br />

tables. Local values of gravity are often subject <strong>to</strong> irregularities which lead <strong>to</strong> errors even when the<br />

corrections here provided are made. It is, therefore, advisable <strong>to</strong> determine the local value of gravity,<br />

from which the correction can be effected in the following manner:<br />

Bt Br <br />

<br />

g g 1 0<br />

g 0<br />

<br />

Br<br />

in which Bt and Br are the true and the observed heights of the barometer, respectively. g 0 is standard<br />

gravity<br />

2<br />

(980 665 cm · s ), and g 1 is the local gravity. It may be noted that for most localities, g 1 is<br />

smaller than g 0 , which makes the correction negative. These corrections compensate the reading <strong>to</strong><br />

gravity at 45 latitude and sea level. (3) Correction for capillary depression of the level of the<br />

meniscus. This varies with the tube diameter and actualheight of the meniscus in a particular case.<br />

Some barometers are calibrated <strong>to</strong> allow for an average value of the latter and approximating the<br />

correction. See table. (4) Correction for vapor pressure of mercury. This correction is usually negligible,<br />

being only 0.001 mm at 20C and 0.006 mm at 40C. This correction is added. See table of<br />

vapor pressure of mercury.<br />

The corrections above do not apply <strong>to</strong> aneroid barometers. These instruments should be calibrated<br />

at regular intervals by checking them against a corrected mercurial barometer.<br />

For records on weather maps, meteorologists cus<strong>to</strong>marily correct barometer readings <strong>to</strong> sea level,<br />

and some barometers may be calibrated accordingly. Such instruments are not suitable for labora<strong>to</strong>ry<br />

use where true pressure under standard conditions is required. Scale corrections should be specified<br />

in the maker’s instructions with the instrument, and are also indicated by the lack of correspondence<br />

between a gauge mark usually placed exactly 76.2 cm from the zero point and the 76.2-cm scale<br />

graduation.

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