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

Fig. 390.—Section through the cistern.

Fig. 391.—Vernier reading, showing gauge point S.

Fig. 392.—Sling case for carrying.

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817.—The Cistern Casing, which is of brass, consists of upper and lower collar pieces, Fig. 390 AA′ and BB′, and their attachments. The upper collar is fixed to the casing tube of the barometer. In the inside of this collar a leather washer is placed, which comes above the boxwood collar on the glass tube D and makes soft contact between these parts. The lower collar has been partly described with the cistern. This has a brass tube E screwed upon it, covering the bag and lower part of the plug of the cistern. The lower closed end of the covering tube is formed into a nut for the adjusting screw F placed in the axis of the tube. There are four bolts or screws GG′ which bring the two collars of the cistern casing towards each other, support the lower part of this casing, and produce a pressure between the boxwood collar on the barometer tube and the top of the glass sighting tube with the intervening rubber collar, so that the mercury at this point is secured.

818.—The Stem, or Barometer Casing Tube, is made of brass, about ¾ inch diameter. This has a slot, of about ¼ inch in width, down two concentrically opposite sides, from near the top of the tube downwards for about 20 inches. The tube is graduated along one open edge next the slot in inches and tenths, these being again subdivided to twentieths, and figured to read from 13 inches to 32 inches of mercury, as shown in detail for the upper part in Fig. 391. The same space is divided into centimetres and millimetres if metrical measure be used. Within the outer tube an inner tube of about 12 inches in length fits telescopically to move with a soft smooth motion. This inner tube carries one vernier at top and one at bottom, Fig. 389, rr′. The top vernier, shown Fig. 391, is placed above a slot in this tube which corresponds with the outer tube, so that the level of the mercury can be seen below the top vernier-piece at S. The verniers are divided into 50, so that, reading into the 20, they give reading 50 × 20, or 1000 to the inch. The inner tube carries a rack about 11 inches long, which moves by a pinion fixed upon a cock-piece, Fig. 389 m, on the outer tube in the same manner as before described for telescope racking, [art. 96]. Two stay-pieces placed over the outer tube hold the slots firmly at an equal opening. A ring is placed at the head of the barometer to suspend it in a room, to be used, if required, as an ordinary meteorological barometer, as shown at the top of Fig. 391.

819.—Mounting of the Barometer.—The barometer is mounted upon a tripod formed of three light tubes with steel points, as shown Fig. 389. These screw into a collar which is packed in the cap of the leather case. The collar has two opposite screws that screw into a second collar, which is also held by two opposite points at right angles to the first. The points of the screws form axes in the manner of a Hook's joint, permitting the barometer to take a vertical position by the superior gravity of its cistern and lower parts.

820.—The Thermometer, shown at Fig. 389 t, has its bulb brought as nearly as possible into contact with the glass tube enclosed in the casing tube. It is commonly divided with both centigrade and Fahrenheit scales. Correct observation of the thermometer is necessary to be made with every observation of the barometer, as the specific gravity of the mercury, and consequently the height of the column, depend partly upon this for its correct determination.

821.—The Packing Case, Fig. 392, is made of solid leather lined with thick felt to fit the barometer. The legs are placed in packings outside the case. In packing for carriage the screw of the cistern is turned nearly home, leaving only sufficient space for any probable expansion of the mercury from increase of temperature. The barometer should always be carried in an inverted position, as this precludes the possibility of air getting into it, and even tends to exclude, by the jarring motion of carrying, any air that may have accidentally become occluded. A strap is attached to the case for holding it over the shoulder.