The following diagrams (half the actual size of the instrument), show the various parts of the instrument, and the method of filling and emptying the reservoir.
Fig. 1 is the instrument complete. A, the mercurial reservoir; B, the observing trough; C, the stop-cock; D, the cylindrical stop.
Fig. 2 is the instrument with the parts of the observing trough removed, which are shown above it. E, rim with glass shade; F, rim without glass shade; G, the glass that floats on the surface of the mercury.
Fig. 3. Position of the instrument while filling the observing trough.
Fig. 4. Position of the instrument while returning the mercury into its reservoir.
In moderate weather the glass G will be quite sufficient protection against the wind, but in gusty weather screw on the rim F, but it must not touch the glass G.
The glass E will protect it from any weather, taking care to level the ground on which the horizon stands.
In filling the observing trough, be careful that the glass cover, E, is screwed on tight; by pressing on the cylindrical stop D (Fig. 1) the mercury flows quickly: the trough half filled, as shown in Fig. 3, is sufficient for ordinary observations; but for very low altitudes the trough must be three-quarters filled or more as found necessary to raise glass G (Fig. 1).