The humidity diagram consists of two distinct sets of curves on the same sheet. One set, the convex curves, is for the determination of relative humidity of wet-and-dry-bulb hygrometer or psychrometer; the other, the concave curves, is derived from the vapor pressures and shows the amount of moisture per cubic foot at relative humidities and temperatures when read at the dew-point. The latter curves, therefore, are independent of all variables affecting the wet-bulb readings. They are proportional to vapor pressures, not to density, and, therefore, may be followed from one temperature to another with correctness. The short dashes show the correction (increase or decrease) which is necessary in the relative humidity, read from the convex curves, with an increase or decrease from the normal barometric pressure of 30 inches, for which the curves have been plotted. This correction, except for very low temperatures, is so small that it may usually be disregarded. The ordinates, or vertical distances, are relative humidity expressed in per cent of saturation, from 0 per cent at the bottom to 100 per cent at the top. The abscissae, or horizontal distances, are temperatures in degrees Fahrenheit from 30 degrees below zero, at the left, to 220 degrees above, at the right. Examples of Use The application of the humidity diagram can best be understood by sample problems. These problems also show the wide range of conditions to which the diagram will apply. EXAMPLE 1. To find the relative humidity by use of wet-and-dry-bulb hygrometer or psychrometer: Place the instrument in a strong circulation of air, or wave it to and fro. Read the temperature of the dry bulb and the wet, and subtract. Find on the horizontal line the temperature shown by the dry-bulb thermometer. Follow the vertical line from this point till it intersects with the convex curve marked with the difference between the wet and dry readings. The horizontal line passing through this intersection will give the relative humidity. Example: Dry bulb 70 deg., wet bulb 62 deg., difference 8 deg.. Find 70 deg. on the horizontal line of temperature. Follow up the vertical line from 70 deg. until it intersects with the convex curve marked 8 deg.. The horizontal line passing through this intersection shows the relative humidity to be 64 per cent. EXAMPLE 2. To find how much water per cubic foot is contained in the air: Find the relative humidity as in example 1. Then the nearest concave curve gives the weight of water in grains per cubic foot when the air is cooled to the dew-point. Using the same quantities as in example 1, this will be slightly more than 5 grains. EXAMPLE 3. To find the amount of water required to saturate air at a given temperature: