Example: Take the values given in example 5. The air at the dew-point contains slightly over 5 grains per cubic foot. At 150 deg. it is capable of containing 73 grains per cubic foot. Consequently, 73-5=68 grains of water which can be evaporated per cubic foot of space at the dew-point when the temperature is raised to 150 deg.. But the latent heat necessary to produce evaporation must be supplied in addition to the heat required to raise the air to 150 deg.. EXAMPLE 8. To find the amount of water evaporated during a given change of temperature and humidity: Example: At 70 deg. suppose the humidity is found to be 64 per cent and at 150 deg. it is found to be 60 per cent. How much water has been evaporated per cubic foot of space? At 70 deg. temperature and 64 per cent humidity there are 5 grains of water present per cubic foot at the dew-point (example 2). At 150 deg. and 60 per cent humidity there are 45 grains present. Therefore, 45-5=40 grains of water which have been evaporated per cubic foot of space, figuring all volumes at the dew-point. EXAMPLE 9. To correct readings of the hygrometer for changes in barometric pressure: A change of pressure affects the reading of the wet bulb. The chart applies at a barometric pressure of 30 inches, and, except for great accuracy, no correction is generally necessary. Find the relative humidity as usual. Then look for the nearest barometer line (indicated by dashes). At the end of each barometer line will be found a fraction which represents the proportion of the relative humidity already found, which must be added or subtracted for a change in barometric pressure. If the barometer reading is less than 30 inches, add; if greater than 30 inches, subtract. The figures given are for a change of 1 inch; for other changes use proportional amounts. Thus, for a change of 2 inches use twice the indicated ratio; for half an inch use half, and so on. Example: Dry bulb 67 deg., wet bulb 51 deg., barometer 28 inches. The relative humidity is found, by the method given in example 1, to equal 30 per cent. The barometric line--gives a value of 3/100H for each inch of change. Since the barometer is 2 inches below 30, multiply 3/100H by 2, giving 6/100H. The correction will, therefore, be 6/100 of 30, which equals 1.8. Since the barometer is below 30, this is to be added, giving a corrected relative humidity of 31.8 per cent. This has nothing to do with the vapor pressure (concave) curves, which are independent of barometric pressure, and consequently does not affect the solution of the previous problems. EXAMPLE 10. At what temperature must the condenser be maintained to produce a given humidity?