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.
It is important that the instrument should be protected not only from the sun’s direct rays, from rain and snow, but also from wind, the currents of which would, by increasing evaporation, cause the wet bulb thermometer to indicate a temperature not strictly due to the hygrometric condition of the atmosphere. For this purpose Thermometer Screens are employed. Illustrations of two forms are shown at Figs. 42 and 43; they should be placed facing the north at a distance of four feet from the ground. Fig. 42 shows the form adopted by the Board of Trade, for marine service, while Fig. 43 shows Mr. Stevenson’s double-louvred screen with perforated bottom, which ensures free ingress and egress of air, the exclusion of snow and rain, and the direct rays of the sun. Professor Wild recommends overlapping segments of sheet zinc for the construction of these screens, as possessing the advantage over wood of becoming sooner in thermic equilibrium with the surrounding air, and thus preventing radiation. Stevenson’s Screen should be erected on legs four feet high, and should stand over grass on open ground. It should not be under the shadow of trees, nor within twenty feet of any wall.
43.
Stevenson’s Thermometer Screen.
Scale about 1/10.