=72. Check instruments.= Humidity is so readily affected by temperature, wind, and pressure, that a knowledge of these factors is essential to an understanding of its fluctuations. Pressure, disregarding daily variation, is taken account of in the tables for ascertaining relative humidity, and is determined once for all when the altitude of a station has been carefully established. The temperature is obtained directly from the dry bulb reading. Its value is fundamental, as the amount of moisture in a given space is directly affected by it; like pressure, it also is taken account of in the formula. The movement of the air has an immediate influence upon moisture by mixing the air of different habitats and layers. So far as the plant is concerned, it has practically the effect of increasing or decreasing the humidity by the removal of the air above it. Thus, while the anemometer can furnish no direct evidence as to the amount of variation, it is of aid in explaining the reason for it. Likewise, the rate of evaporation as indicated by a series of atmometers, affords a ready method of estimating the comparative effect of humidity in different habitats. Potometers and other instruments for measuring transpiration throw much light upon humidity values. Since they are concerned with the response of the plant to humidity, they are considered in the following chapter. =73. Humidity tables.= To ascertain the relative humidity, the difference between the wet and dry bulb readings is obtained. This, with the dry bulb temperature, is referred to the tables, where the corresponding humidity is found. A variation in temperature has less effect than a variation in the difference; in consequence, the dry bulb reading is expressed in the nearest unit, and the difference reckoned to the nearest .5. The humidity varies with the air pressure. Hence, the altitude must be determined for the base station, and for all others that show much change in elevation. Within the ordinary range of growing-period temperatures, the effect of pressure is not great. For all ordinary cases, it suffices to compute tables for pressures of 30, 29, 27, 25, and 23 inches. The following table indicates the decrease in pressure which is due to altitude. ALTITUDE PRESSURE Feet Meters Inches Centimeters 0 0 30 76 910 277 29 73.5 1850 574 28 71 2820 860 27 68.5 3820 1165 26 66 4850 1477 25 63.5 5910 1792 24 61 7010 2138 23 58.5 8150 2485 22 56 9330 2845 21 53.5 10550 3217 20 51 13170 4016 18 46 16000 4880 16 41