Find on the top line (100 per cent humidity) the given temperature; the concave curve intersecting at or near this point gives the number of grains per cubic foot. (Interpolate, if great accuracy is desired.) EXAMPLE 4. To find the dew-point: Obtain the relative humidity as in example 1. Then follow up parallel to the nearest concave curve until the top horizontal (indicating 100 per cent relative humidity) is reached. The temperature on this horizontal line at the point reached will be the dew-point. Example: Dry bulb 70 deg., wet bulb 62 deg.. On the vertical line for 70 deg. find the intersection with the hygrometer (convex) curve for 8 deg.. This will be found at nearly 64 per cent relative humidity. Then follow up parallel with the vapor pressure (concave) curve marked 5 grains to its intersection at the top of the chart with the 100 per cent humidity line. This gives the dew-point as 57 deg.. EXAMPLE 5. To find the change in the relative humidity produced by a change in temperature: Example: The air at 70 deg. Fahr. is found to contain 64 per cent humidity; what will be its relative humidity if heated to 150 deg. Fahr.? Starting from the intersection of the designated humidity and temperature coordinates, follow the vapor-pressure curve (concave) until it intersects the 150 deg. temperature ordinate. The horizontal line then reads 6 per cent relative humidity. The same operation applies to reductions in temperature. In the above example what is the humidity at 60 deg.? Following parallel to the same curve in the opposite direction until it intersects the 60 deg. ordinate gives 90 per cent; at 57 deg. it becomes 100 per cent, reaching the dew-point. EXAMPLE 6. To find the amount of condensation produced by lowering the temperature: Example: At 150 deg. the wet bulb reads 132 deg.. How much water would be condensed if the temperature were lowered to 70 deg.? The intersection of the hygrometer curve for 18 deg. (150 deg.-132 deg.) with temperature line for 150 deg. shows a relative humidity of 60 per cent. The vapor-pressure curve (concave) followed up to the 100 per cent relative humidity line shows 45 grains per cubic foot at the dew-point, which corresponds to a temperature of 130 deg.. At 70 deg. it is seen that the air can contain but 8 grains per cubic foot (saturation). Consequently, there will be condensed 45 minus 8, or 37 grains per cubic foot of space measured at the dew-point. EXAMPLE 7. To find the amount of water required to produce saturation by a given rise in temperature: