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.