Dial (or Wheel) Barometer.—The tube intended for this barometer should be very regular in the bore. It should be thirty-nine inches long. Close it at one end, and bend it like the letter U at about thirty-two inches from the sealed extremity. See [pl. 2], fig. 5, and Graduation of the Dial Barometer.
Syphon Barometer.—Make use of such a tube as might be employed for a Cistern Barometer; solder to its open end a cylindrical or spherical reservoir, and bend the tube close to the point of junction in such a manner as to bring the cylinder parallel with the tube. If the reservoir is to be closed with a cover of leather, cut off the remaining point of the cylinder, slightly widen the orifice, and then border it. If no leather is to be applied, but the point of the cylinder left open, it is necessary, after the introduction of the mercury, to draw off the point abruptly, and to leave an opening so small that mercury cannot pass by it. [Pl. 2], fig. 6.
Stop-Cock Barometer.—This differs from the preceding barometer only by having a stop-cock mounted in iron between the reservoir and the tube.
Compound Barometers.—Blow a bulb at each end of a barometer tube of about thirty-three inches in length. Solder a small and almost capillary tube to the point which terminates one of the bulbs, and bend the great tube very near this bulb. This must be done in such a manner that the centre of one bulb shall be thirty inches from the centre of the other bulb. Introduce a quantity of mercury sufficient to fill the great tube and half the two bulbs; fill the remaining space in the last bulb with alcohol.
You may give a different disposition to this instrument. Divide a barometer tube into two, three, or four pieces, and reunite the pieces by intermediate capillary tubes, so as to form a series of large and small tubes, soldered alternately the one at the end of the other. Then communicate to this compound tube the form exhibited by [pl. 3], fig. 25, and join, at each superior bend, a little tube, for the convenience of easily filling the instrument with mercury: seal these tubes as soon as the mercury is introduced. The graduation of compound barometers is made by bringing them into comparison with a good standard barometer. After taking two or three fixed points, it is easy to continue the scale.
Gay Lussac’s Barometer.—Take a tube which is very regular in the bore, four-tenths of an inch in diameter, and thirty-five inches and a half in length. Seal one of its extremities and draw out the other; then cut the tube at about two-thirds of its whole length from the sealed end, and reunite the two pieces by means of a capillary tube soldered between them, the whole being kept in a line. See [pl. 2], fig. 1. Pierce laterally the part of the tube which is drawn out, at some inches from the base of the point, and force the margin of the hole into the interior of the tube, by means of a conical point of metal, in such a manner as to form a little sunk funnel, of which the orifice must be very small. After having introduced the proper quantity of mercury into the instrument, boil it, and assist the disengagement of the bubbles of air by agitating a fine iron wire within the tube. Then remove the part of the tube which was drawn out, by sealing the end of the wide part. Give to the whole instrument the curvature indicated by [pl. 2], fig. 3.