=79. Casella's Mercurial Minimum Thermometer.=--The general form and arrangement of this instrument is shown in fig. 60. A tube with large bore, _a_, has at the end a _flat glass diaphragm_ formed by the abrupt junction of a small chamber, _b c_, the inlet to which at _b_ is larger than the bore of the indicating tube. The result of this is that on setting the thermometer, as described below, the contracting force of the mercury in cooling withdraws the fluid in the indicating stem only; whilst on its expanding with heat, the long column does not move, the increased bulk of mercury finding an easier passage into the small pear-shaped chamber attached. [Illustration: Fig. 60.] We believe that a small speck of air must be confined in the chamber, _b c_, to act as a spring to start the mercury from the chamber in the act of setting the thermometer. Were this air not present, the mercury would so adhere to the glass that no amount of shaking could induce it to flow from the chamber. _To set the Instrument_, place it in a horizontal position, with the back plate, _d_, suspended on a nail, and the lower part supported on a hook, _e_. The bulb end may now be gently raised or lowered, causing the mercury to flow slowly until the bent part, _a_, _is full_ and the chamber, _b c_, _quite empty_. At this point the flow of mercury in the long stem of the tube is arrested, _and indicates the exact temperature_ of the bulb or air at the time. On an increase of temperature the mercury will expand into the small chamber, _b c_; and a return of cold will cause its recession from this chamber only, until it reaches the diaphragm, _b_. Any further diminution of heat withdraws the mercury down the bore to whatever degree the cold may attain, where it remains until farther withdrawn by increased cold, or till re-set for future observation. MAXIMA AND MINIMA THERMOMETERS. =80. Rutherford's= arrangement for obtaining a complete instrument for the registration of heat and cold was simply mounting a maximum thermometer and a minimum thermometer upon the same frame or slab. Thus constructed, they are often called "day and night" thermometers, though somewhat inappropriately; for in temperate climates the temperature of the night sometimes exceeds that of the day, notwithstanding the reverse is the general law of temperature. Fig. 61 will explain the arrangement of Rutherford's day and night thermometer. [Illustration: Fig. 61.]