The instrument, as shown at page 48, consists of two thermometers attached to a support, which may be either slate or wood. The bulb of one of the thermometers has some thin muslin tied over it, and is kept moist by the capillary action of a thread dipping into a cistern of water placed underneath. It will be obvious that the amount of evaporation will be in proportion to the dryness of the air, and that the differences of temperature indicated by the two thermometers will be greatest when the atmosphere is dry, and least when the air is damp. [Illustration: 42. Board of Trade Thermometer Screen. Scale about 1/9.] HYGROMETER PRECAUTIONS. Hygrometers should be exposed in the shade free from air-currents. The covering of the wet bulb must be very thin. The supply of water must be carefully regulated. The bulb must be constantly _moist_, yet not _too wet_. The supply of water must be ample in dry weather. In damp weather water must not drip from the wet bulb. Water reservoir should be as far as possible from the dry bulb. Dry bulb must never receive moisture from any source. Use distilled, rain, or softest water procurable, for wet bulb. When lime deposits from use of hard water change muslin and worsted. Replenish reservoir after, or long before, taking an observation. Well wash muslin and worsted before using. Also wash occasionally while in use. Change muslin twice a month or according to condition. Dust and blacks must not be allowed to accumulate on muslin. ⎧ When wet bulb is frozen, wet with ice-cold water by brush. ⎪ The water will first freeze, then cool to air-temperature. ⎨ After which wet bulb falls a trifle lower than dry one. ⎩ Then temperature of evaporation may be noted. ⎧ In thick fog wet bulb reads _above_ dry bulb. ⎪ In cold calm weather, wet bulb reads _above_ dry bulb. ⎨ This is owing to the air being perfectly saturated. ⎪ Covered bulb cannot therefore show temperature as well as uncovered. ⎩ In such cases both readings are assumed to be identical.