Uniformity of Calibre.—The diameter of that part of the tube through which the oscillations of the mercury will take place is very carefully examined to insure uniformity of calibre, and only those tubes are used which are as nearly as possible of the same diameter throughout. The size of the bore should be marked on the frame of the barometer in tenths and hundredths of an inch. A correction due to capillary action, and depending on the size of the tube, must be applied to the readings.
5. Correction due to Capillarity.—When an open tube of small bore is plunged into mercury, the fluid will not rise to the same level inside as it has outside. Hence, the effect of capillary action is to depress the mercurial column; and the more so the smaller the tube. The following table gives the correction for tubes in ordinary use:—
| Diameter of tube. | Depression, in boiled tubes. | Depression, in unboiled tubes. | ||
| INCH. | INCH. | INCH. | ||
| 0·60 | 0·002 | 0·004 | ||
| 0·55 | 0·003 | 0·005 | ||
| 0·50 | 0·003 | 0·007 | ||
| 0·45 | 0·005 | 0·010 | ||
| 0·40 | 0·007 | 0·015 | ||
| 0·35 | 0·010 | 0·021 | ||
| 0·15 | 0·044 | 0·029 | ||
| 0·10 | 0·070 | 0·041 | ||
| 0·30 | 0·014 | 0·058 | ||
| 0·25 | 0·020 | 0·086 | ||
| 0·20 | 0·029 | 0·140 |
This correction is always additive to the observed reading of the barometer.
6. Correction due to Temperature.—In all kinds of mercurial barometers attention must be given to the temperature of the mercury. As this metal expands and contracts very much for variations of temperature, its density alters correspondingly, and in consequence the height of the barometric column also varies. To ascertain the temperature of the mercury, a thermometer is placed near the tube, and is sometimes made to dip into the mercury in the cistern. The freezing point of water, 32°F., is the temperature to which all readings of barometers must be reduced, in order to make them fairly comparable. The reduction may be effected by calculation, but the practical method is by tables for the purpose; and for these tables we refer the reader to the works mentioned at the end of this book.
7. Correction due to Height above the Half-tide Level.—Further, in order that barometrical observations generally may be made under similar circumstances, the readings, corrected for capacity, capillarity, and temperature, should be reduced to what they would be at the sea-level, by adding a correction corresponding to the height above the mean level of the sea, or of half-tide. For practical purposes of comparison with barometric pressure at other localities, add one-tenth of an inch to the reading for each hundred feet of elevation above the sea. For scientific accuracy this will not suffice, but a correction must be obtained by means of Schuckburg’s formula, or tables computed therefrom.
Fig. 4.
Fig. 5.
