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. * * * * *