. Annual report of the Board of Regents of the Smithsonian Institution. Smithsonian Institution; Smithsonian Institution. Archives; Discoveries in science. 522 A UNIVERSAL METEOROGRAPH, As regards temperature, any of the instruments may be used which depend upon the expansion of solid bodies, so as to afford the means of moving a lever. Such are Breguet's thermometer, Secchi's thermo- graph—founded on the linear expansion of a copper rod, and Dr. Krecke's metallic thermometer—founded on the different rates of expansion of zinc and glass.* The air thermometer may also be employed, the lever bei
. Annual report of the Board of Regents of the Smithsonian Institution. Smithsonian Institution; Smithsonian Institution. Archives; Discoveries in science. 522 A UNIVERSAL METEOROGRAPH, As regards temperature, any of the instruments may be used which depend upon the expansion of solid bodies, so as to afford the means of moving a lever. Such are Breguet's thermometer, Secchi's thermo- graph—founded on the linear expansion of a copper rod, and Dr. Krecke's metallic thermometer—founded on the different rates of expansion of zinc and glass.* The air thermometer may also be employed, the lever being actuated by a float on the surface of the mercury in the manometer. The ordinary mercurial thermometer alone is inadmissible * As, to my knowledge, Krecke's metallic thermometer has never heen descrihed, and as it deserves a more general use in meteor- ological observatories, on account of its simplic- ity, accuracy, and ready adaptation to automatic registry, I give here the principle of its construc- tion. Two glass tubes, o o (Fig. I), about 5 feet long and 1 inch in diameter, have their ends closed with two iron plates, A and B, the lower one, B, being fastened firmly to a wall, while the other is so attached to the wall as to permit a slight dis- placement. To the piece A is connected a zinc bar, C C, and to the piece B a similar bar, D D'. These bars are otherwise entirely free throughout the rest of their length, but, in order to prevent them from bending, a ring is placed near the free end of each, so as to slip without sensible friction on the glass tubes o o. In order that the zinc bars may quickly assume the temperature of the air they must not be too thick, and yet they must have sufficient rigidity, which is secured by making them trough-shaped. At the free end of the bar D D' a steel support, p, is fixed, against which rests the brass lever R M, somewhat like the arm of a balance. The arms D' M and D' R of this lever are unequal, and there is a movab
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