The dials are generally made of metal silvered over or enamelled, but porcelain may be used. If the circumference of the pulley, or "wheel," be two inches, it will revolve once for an alteration of level amounting to two inches in each tube, or four inches in the height of the barometric column; and as the dial may be from twenty to thirty-six inches in circumference, five to nine inches on the graduated scale corresponds to one inch of the column; and hence the sub-divisions are distinctly perceptible, and a vernier is not necessary. The motion of the pointer alone is visible; and a mahogany, or rosewood, frame, supports, covers, and renders the instrument ornamental and portable. In the back of the frame is a hinged door, which covers the cavity containing the tube and fixtures. The dial is covered by a glass in a brass rim, similar to a clock face. A brass index, working over the dial, moveable by a key or button, may be applied, and will serve to register the position of the hand when last observed. These instruments are usually fitted with a thermometer, and a spirit level; the latter for the purpose of getting the instrument perfectly vertical. They sometimes have, in addition, a hygrometer, a sympiesometer, an aneroid, a mirror, or a clock, &c., singly or combined. The frame admits of much variety of style and decoration. It may be carved or inlaid. The usual adjustment of scale is suited for localities at no considerable elevation above the sea. Accordingly, being commercial articles, they have been found frequently quite out of place. When intended for use at high elevations, they should have a special adjustment of scale. As household instruments they are serviceable, and ornamental. But the supply-and-demand principle upon which they are sold, has entailed upon those issued by inferior makers a generally bad adjustment of scale. The illustrations are those of ordinary designs. [Illustration: Fig. 21.] [Illustration: Fig. 22.] [Illustration: Fig. 23.] Dial barometers required for transmission to distant parts, as India and the Colonies, are furnished with a steel stop-cock, to render them portable more effectually than can be done by the method of _plugging_ the tube. 32. STANDARD SYPHON BAROMETER.