following tables give the capacity of air for moisture at different temperatures, and the percentage of saturation as shown by the wet and dry thermometer. At Greenwich, the mean humidity for the year is 82 per cent.; or for the day-time only 76 per cent., varying from 62 in summer to 86 in winter:-- Table I.--Capacity of Air for Moisture. ───────────┬──────────────────┬──────────────────────┐ │ Weight in Pounds │ Weight in Pounds of │ Temp. Fahr.│ of a Cub. Ft. of │ Moisture contained in│ │ Dry Air. │ a Cub. Ft. of │ │ │ Saturated Air. │ ───────────┼──────────────────┼──────────────────────┤ 32° │ ·0807 │ ·000304 │ 42 │ ·0791 │ ·000440 │ 52 │ ·0776 │ ·000627 │ 62 │ ·0761 │ ·000881 │ 72 │ ·0747 │ ·001221 │ 82 │ ·0733 │ ·001667 │ 92 │ ·0720 │ ·002250 │ 102 │ ·0707 │ ·002997 │ ───────────┴──────────────────┴──────────────────────┘ Table II.--Hygrometer Table. ───────────┬───────────────────────────────────────────────┐ Temperature│ Degrees between Wet and Dry Thermometers. │ of Air. ├───┬───┬───┬───┬───┬───┬───┬───┬───┬───┬───┬───┤ │ 1 │ 2 │ 3 │ 4 │ 5 │ 6 │ 7 │ 8 │ 9 │ 10│ 11│ 12│ ───────────┼───┼───┼───┼───┼───┼───┼───┼───┼───┼───┼───┼───┤ 32° F. │ 87│ 75│ ..│ ..│ ..│ ..│ ..│ ..│ ..│ ..│ ..│ ..│ 42 │ 92│ 85│ 78│ 72│ 66│ 60│ 54│ 49│ 44│ 40│ 36│ 33│ 62 │ 94│ 88│ 82│ 77│ 72│ 67│ 62│ 58│ 54│ 50│ 47│ 44│ 82 │ 95│ 90│ 85│ 80│ 76│ 72│ 68│ 64│ 60│ 57│ 54│ 51│ │ │ │ Per cent. of moisture, saturation being 100.│ ───────────┴───────────────────────────────────────────────┘ As regards the heat consumed in evaporation; it requires about 1000 times as much heat to convert 1 lb. of water into vapour, as it does to raise the temperature of the same quantity 1° F. At least as much heat as this must be supplied if the air which has been used in drying is to retain the same temperature it had at the outset, and therefore if a turret is to keep at a higher temperature than the air, which is necessary to create a draught, this is the minimum amount of heat which must be supplied per pound of water to be evaporated. In practice much more will be needed. The following table shows the heat given out by different sizes of pipes at different temperatures, and steam pressures, in units equal to the heat required to raise 1 lb. of water 1° F., and the cubic feet of air which they will heat.[V]