Page:Handbook of Meteorology.djvu/229

 The hygrodeik is a form of hygrometer in which the tabulated matter is shown on a card ruled with ordinates and co-ordinates for the convenience of reading. An index fastened by a hinge joint at the top carries also a sliding point. By the adjustment of these the relative humidity is read from the tabulated figures. The experience of nearly a century has shown the usefulness of this instrument for indoor purposes.

Unless “coaxed” by fanning, dry -wet-bulb calculations are subject to error, the nature of which is obvious. The use of an ordinary fan—or, better, an electric fan—on the bulbs will give much more accurate readings.

The sling psychrometer obviates this difficulty. The two thermometers of this instrument are made fast to a metal strip which whirls upon a pivoted handle. The covered bulb is dipped in water of the same temperature as the air and whirled on the pivot until the temperature of the wet bulb ceases to lower. Ordinarily, about twenty seconds are required to obtain a correct reading.

The whirling table is now generally employed where systematic observations are made. The geared mechanism used in whirling the thermometers does not give more accurate results than the sling psychrometer, but it affords an easier method of stimulating evaporation, and the thermometers are not so likely to be broken.

When the humidity of the air is near the point of saturation, determinations made at the same time may vary several points; and, unless a sling psychrometer or a whirling apparatus is used, the determinations are pretty certain to vary. The thermometer scales of the best psychrometers are graduated to half-degrees and may be read to quarter-degrees. This conduces materially to accuracy.

The chief source of inaccuracy, however, is the covering of the wet-bulb thermometer. No matter what the material of which it is constructed may be, sooner or later it becomes hard and loses its capillarity. It is no longer of use and should be thrown away. If it shows signs of discoloration it should be thrown away also, for its usefulness is gone. Tubular wicks are now much used and are kept by dealers in meteorological instruments. If any doubt as to the cleanliness of a wick exists, it should be boiled briskly in water and, when dry, soaked in