[Illustration: FIG. 158.--The hygrodeik. A form of hygrometer in which relative humidity is found directly from a diagram.] =The Hygrodeik.=--In Fig. 158 is shown a form of hygrometer known as a hygrodeik, by means of which atmospheric humidity may be determined without the use of the tables. In the figure the wet-bulb and dry-bulb thermometers are easily recognized. A glass water bottle _W_ is held to the base of the instrument by spring clips which permit its removal to be filled with water. Between the thermometers is a diagram chart from which the atmospheric humidity is taken. An index arm, carrying a movable pointer _P_, permits the instrument to be set for any observed thermometer readings. The index is really a graphical method of expressing the figures given in the table on pages 202-203. In the picture the wet-bulb thermometer reads 65°, the dry-bulb thermometer 77°. To determine the relative humidity under these conditions the movable arm is swung to the left and the pointer _P_ placed on the left-hand scale at the line 65°. The arm is then swung to the right until the pointer touches the downward curving line beginning at 77°, the dry-bulb reading. The lower end of the arm _H_ now points to the relative humidity, where 52 per cent. is indicated by the scale at the bottom of the index. The same result is obtained from the table of Relative Humidity. The readings of the thermometers in the figure give a difference in temperature of 12°, the dry-bulb thermometer reads 77°. Referring this data to the humidity table, the column marked 12, for the depression of the wet-bulb thermometer and opposite 77° in the air temperature column, is found 53 which indicates the per cent. of saturation. The hygrodeik gives further the temperature of the dew-point, on the scale to the right; and the absolute humidity may be found by following the upward curving line nearest the pointer, at the end of which line is given the value in grains of moisture per cubic foot. The hygrodeik or other instrument of the kind is very largely used in places where relative humidity is regularly observed by those of limited experience, as in school-rooms, auditoriums, etc. Such records are not intended to be perfectly accurate and the readings of the hygrodeik are very well-suited for the purpose. In using the hygrometer and the hygrodeik the instruments are stationary; they are usually hung on the wall in a convenient location for observation and are placed to avoid accidental drafts in order that the conditions surrounding the observation may be the same at all times. The evaporation which takes place from the wet bulb is due to natural convection and does not always reach the maximum amount. The evaporation is furthermore influenced by accidental variations and consequently the results cannot be considered exact. [Illustration: FIG. 159.--Psychrometer of U. S. Weather Bureau type; for accurate determination of atmospheric humidity.]