The centigrade differs from Reaumur’s only in having the space between the boiling and freezing point of water divided into 100 parts instead of 80. What are called “register thermometers” have two bulbs, stalks, and scales, on the same instrument (fig. 3); one bulb is filled with mercury, and the other with colored spirit. In each stalk a piece of enamel, about half-an-inch long and fitting the cavity, is introduced; the one in the mercury is to register the highest, and that in the spirit to register the lowest degree of heat. They act in the following manner:--The spirit, being very liquid or thin in its nature, wets the enamel and passes by it when it rises in the stalk, so that the elevation of temperature does not affect its position, but when the spirit sinks down it drags the enamel with it, thus registering the lowest temperature, so that the distance the enamel is found down the stalk indicates how low the spirit may have descended in any particular time, say a night. With respect to the mercury, it is not of a nature to adhere to the enamel, and therefore instead of passing it pushes it up in the stalk as it rises, but on descending leaves it behind, the height at which the enamel is found up the stalk indicating the highest point to which the mercury had risen, and consequently the highest temperature. To adjust the instrument, a slight tap or shake will make the index in the spirit tube fall to the surface of the spirit, where it is held by the adhesive quality of the liquid, and by the same process that in the mercurial stalk will fall to the surface of the mercury, but will not penetrate it, owing to its great density. BAROMETERS. Illustration: FIG. 1. (‡ BENT GLASS TUBE.) Illustration: FIG. 2. (‡ TUBE IMMERSED IN MERCURY.) Illustration: FIG. 3. (‡ WEATHER GLASSES.)