Surveying and Levelling Instruments, Theoretically and Practically Described. / For construction, qualities, selection, preservation, adjustments, and uses; with other apparatus and appliances used by civil engineers and surveyors in the field.

by William Ford Stanley

811.—The mean height of the barometrical column in Great Britain, at sea level at the temperature of 32° Fahr., is about 29·95 ins. A cubic inch of mercury at this temperature weighs 0·48967 lbs. Therefore

29·95 × 0·48967 = 14·66 lbs.

gives the mean pressure of the atmosphere on each square inch of surface of the earth in this latitude. Nearer the tropics the pressure is greater, near the poles less. It can be shown that as the heights ascended by the barometer increase in arithmetical progression, the pressure upon the mercury diminishes in geometrical progression.

812.—Mountain Barometer.—The barometer used for measuring altitudes, to which the above term has been applied, is now made only upon Fortin's plan, in which the bottom of the cistern wherein the glass tube is plunged is made of fine, close-grained leather, the best for the purpose being a stout kid. The pores of the leather must be sufficiently fine not to admit of the escape of the mercury, and yet at the same time sufficiently soft and pliable to transmit the exterior pressure of the air. Fortin's construction permits the cistern to be closed entirely secure from leakage of the mercury, in whatever position the barometer may be placed. The closing is effected by means of an adjusting screw, [Fig. 390] F, which by its pressure decreases the capacity of the cistern and forces the mercury up the tube, or adjusts it to a given height, so that the scale of the barometer may be read correctly from a given point X placed within the cistern. To prevent injury to the tube the adjusting screw is made of a length just sufficient to force the mercury to fill it, so that when it is closed home there is no jar or percussion of the mercury in carrying the barometer. The details of the mountain barometer may be best followed by the illustrations.

813.—The Glass Tube is made of mild flint glass thoroughly annealed and sufficiently stout to resist all the strain and percussion that may occur with fair usage. One end of the tube is slowly sealed by the blow-pipe, so that the closed end may be as strong as the other parts.

814.—Mercury—Filling the Barometer Tube.—The mercury of commerce is generally impure, and it contains occluded air. For standard and mountain barometers the mercury should be distilled in an iron apparatus, at just its boiling heat, leaving about one-sixth of the mercury in the still. The tube, which should be perfectly clean, is left about 12 inches too long for the barometer. It is charged with clean mercury for about 36 inches in height. It is then boiled in a special circular charcoal stove, in the centre of which there is a vertical iron tube of 2 inches diameter. The barometer tube is introduced from the bottom of the stove, to heat about 4 inches of the top of the mercury only. The tube remains in this position till the mercury boils. It is then elevated for another 4 inches and again brought to boiling point, and so on until the end of the tube is reached. Under this process the air and some impurities rise to the surface of the mercury, and the tube is considered to be properly boiled. The end of the tube is then cut off to its proper length and inserted in the cistern, in which there is left sufficient clean mercury to complete the barometer.

815.—The lower part of the barometer tube, after it is filled, is attached to a thin boxwood socket of about an inch in depth by means of hot thin glue. The socket piece is afterwards bound over with sewing silk, which is again covered with glue, and is finally varnished so as to form an elastic, secure fitting upon the glass. The socket-piece is secured to a wide boxwood collar, Fig. 390, D. Upon the under side of the collar an ivory gauge peg X is inserted, which forms the index point for reading the surface of the mercury in the cistern upon the Fortin principle.

816.—The Cistern.—The glass sighting tube, Fig. 390 H, of the cistern, through which the mercury and gauge point X are visible, is made about 1½ inches long and from 1 inch to 1½ inches internal diameter, the glass being 1/8 inch to 1/5 inch in thickness, ground square at its ends. The upper end of the glass fits upon the boxwood collar D, with the interval of an indiarubber band to make the fitting air-tight. The lower end of the glass tube fits upon the boxwood collar I, with an interval of a turned leather collar. The boxwood collar prolonged forms the lower part of the cistern. This has a second boxwood collar screwed upon it, to which the leather bag E is attached by silk and glue. A stout leather capping plug is glued upon the lower end of the bag, upon which the boxwood cap of the adjusting screw F presses for adjustment of the mercury, or to close the tube.

Fig. 389.—Mountain barometer erected for use.