=Thermometers.=—The =maximum thermometer= may be on Negretti and Zambra’s, or on Phillips’ principle. In the former (Fig. 49) the bore of the tube is reduced in section near the bulb (A) in such a way that while the expanding mercury forces its way into the tube, the column of mercury breaks off on contraction, so that its upper limit shows the highest temperature that has been reached. The thermometer is set by holding it bulb downwards and shaking to make the mercurial column continuous. It is mounted in the screen horizontally (Fig. 51). [Illustration: FIG. 49. NEGRETTI AND ZAMBRA’S MAXIMUM THERMOMETER.] The =minimum thermometer= chiefly used is Rutherford’s. It contains spirit in which is an immersed index (A, Fig. 50). With a falling temperature the spirit draws the index along with it; but on rising again, the spirit passes the index, leaving it at the lowest point to which it has been drawn. Thus the end farthest from the bulb registers the minimum temperature. The instrument is set by raising the bulb and allowing the index to slide to the end of the column of spirit. The thermometer must be firmly fixed and mounted quite horizontally. [Illustration: FIG. 50. MINIMUM THERMOMETER.] =Thermometer Screen.=—The above thermometers, as well as the dry and wet bulb thermometers are mounted in a Stevenson’s screen (Fig. 51). This is a doubled-louvred box through which the air can pass freely, but the sun cannot enter. The horizontal position of the maximum and minimum and the vertical position of the dry and wet bulb thermometers are shown in Fig. 51. Three additional thermometers are usually included in a well-organised meteorological station. A =minimum thermometer= placed on the grass gives the lowest temperature on the grass, which is often considerably lower than that of the neighbouring gravel walk. This record is chiefly useful for agricultural purposes. The =earth thermometer= chiefly used is shown in Fig. 52. It consists of a sluggish thermometer mounted in a short weighted stick attached to a strong chain, and of a stout iron pipe which is drawn out at the bottom to a point and driven into the earth, usually to a depth of 4 feet. =Solar radiation= is measured by black-bulb and light-bulb thermometers in _vacuo_, which are mounted on a post 4 feet above the ground and record the maximum temperature. =Humidity= in the air is measured by direct or indirect hygrometers. Of the former Dines’, Daniell’s, and Regnault’s are the best known, but as they are not used in observations acknowledged by the Royal Meteorological Society, the reader may be referred to their description in books on physics. The indirect hygrometer which is universally employed in this country is that furnished by the =dry and wet bulb thermometers=. In frosty weather they require much attention, and then a Saussure’s hair hygrometer may be used as supplementary. The general arrangement of the dry and wet bulb thermometers is shown in Fig. 53.