There is a modification of this latter form produced by the addition of a supplementary chamber just outside the bulb and _over_ the column, from which, as expansion proceeds, the mercury flows by gravitation, but into which it cannot return until, as in the other forms, the instrument is readjusted for a new observation, by unhooking the bulb end and lowering it until the mercury flows into its place. [Illustration: 11. Self-registering Minimum Thermometer. Scale about 1/5.] Self-registering Minimum Thermometers are of two kinds,—spirit and mercurial. Fig. 12 shows one of Rutherford’s Alcohol Minimum Thermometers, which will be seen to consist of a bulb and tube attached to a scale, which latter may be either of wood, glass, or metal. The tube contains an index of black glass. [Illustration: 12. Self-registering Minimum Thermometer. Scale about 1/5.] The Thermometer is “set” for observation by slightly raising the bulb end until the index slides to the extreme end of the column of spirit. It is then suspended in the shade with the bulb end a little lower than the other. The contraction of the spirit consequent on a fall of temperature draws the index back, but a subsequent expansion does not carry it forward, it remains at the lowest point to which the spirit has contracted to register the minimum temperature. A very useful modification of this instrument is made for gardeners and general horticultural purposes, in which the scale is of cast zinc with raised figures, which being filed off flush after the whole has been painted of a dark colour are easily legible at a little distance. The advantage of alcohol for the indication of _very_ low temperatures is that it has never been frozen.[5] [Footnote 5: Mercury freezes at -39° F.] Fig. 13 shows a set of Maximum and Minimum and Wet and Dry Bulb Thermometers, with incorrodible porcelain scales, suspended on a mahogany screen. Instruments of this quality are generally engine-divided on the stem, and if, in addition to this, they are verified by comparison with standard instruments at the Kew Observatory, they may be regarded as standards, and employed for accurate scientific observations. [Illustration: 13. Standard Set of Instruments on Screen. Scale about 1/6.]