822.—Reading the Barometer.—It will be observed that the mercury against the sides of the tube presents an upward curved appearance, due to the resistance of the glass to perfect contact, and the cohesion of the mercury in what is termed capillary action. This beading, as it is termed, varies according to whether the mercury is rising or falling. It is always necessary before taking an observation to raise the mercury, by turning the screw F, until its surface just touches the peg X, to make observations uniform. The reading is taken by slowly lowering the index-piece by means of the milled screw until light is just excluded between the fore and back index surfaces, as shown Fig. 391 at S, at the highest point of the surface of the mercury. The inches, tenths, and half-tenths (·05) are read on the scale, and the thousandths on the vernier. Thus, suppose the scale reads 26·45 and the vernier 25 = 25 thousandths, the reading will be—
| 26·45 |
| ·025 |
| 26·475 |
For altitude the upper and lower stations are taken, and the difference subtracted for difference of barometrical scale.
823.—Difference in Altitude in feet taken from Barometrical Inches.—Complete barometrical tables for this comparison will be found in Molesworth's and other pocket-books in use by all engineers. It is therefore unnecessary to occupy our space with them. A very approximate rule may be given, which was proposed by Mr. R. Strachan in the Meteorological Magazine, as follows:—
"Read the barometer to the nearest hundredth of an inch; subtract the upper reading from the lower, leaving out the decimal point; and then multiply the difference by 9, which gives the elevation in feet. Thus:—
| Lower | station | 29·25 | inches |
| Upper | " | 28·02 | " |
| 123 | |||
| 9 | |||
| Elevation | 1107 | feet." |
824.—Capillarity.—For meteorological observations a quantity must be added to the reading equal to the resistance of the tube in capillary action to the rise of the mercury. This is greater in an unboiled tube than in one in which the mercury is boiled. For altitude measurements with a single barometer, or by two barometers with equal tubes, it may be neglected, as it will be equal in all parts of the tube. Where two barometers of different bores are used, the following table gives the correction:—
| Diameter of Tube in Inches | ·6 | ·55 | ·5 | ·45 | ·4 | ·35 | ·3 | ·25 | ·2 | ||
| Unboiled | Tube, | Inches | ·004 | ·005 | ·007 | ·01 | ·014 | ·02 | ·025 | ·04 | ·059 |
| Boiled | " | " | ·002 | ·003 | ·004 | ·005 | ·007 | ·01 | ·013 | ·02 | ·029 |
825.—Temperature Correction.—As the mercury increases in temperature it becomes specifically lighter, therefore rises higher in the tube under equal atmospheric pressure. The temperature is indicated by the thermometer, shown at [Fig. 389] t. The expansion of mercury for 1° Fahr. is 0·000101; but the brass tube also expands 0·0000104, and it is the difference between the two expansions that we require, the mercury expanding about 7·15 more than the brass. If we subtract from the reading ·00014 of the observed altitude for every degree of Fahrenheit above 32°, the correction will be practically very near. Thus for a single reading—thermometer, 52° Fahr.; barometer, 30 inches