0.464 × 2.44 × 1200 = 1356.00 B.t.u. Of this last amount of heat, approximately 50 per cent. is recovered as the gases pass through the furnace. The total loss of heat due to the evaporation of the water is Raising temperature from normal to 212° 395 B.t.u. Evaporation 2,366 B.t.u. Changing temperature of vapor, less 50 per cent. 678 B.t.u. ------------ Total heat loss 3,439 B.t.u. In the 100 pounds of coal under consideration, there is 100 pounds, less 2.44 pounds of water, or 97.56 of dry coal, each pound of which contains 13,732 B.t.u. as given by the table on page 193. This gives 97.56 × 12,682 = 1,339,753 = practically 1,340,000 B.t.u. From this quantity is subtracted the loss of heat, 3439. 1,340,000 - 3439 = 1,336,561 B.t.u. This represents the total available heat in 100 pounds of coal. If this quantity is now divided by the cost of 100 pounds of coal at $7.25 per ton, the result, 3,564,000 B.t.u., will be the available heat bought for $1 as given in column 7 of the table. CHAPTER X ATMOSPHERIC HUMIDITY The physical effect of atmospheric humidity has come to be recognized by all who deal in problems of house heating, sanitation and hygiene. The difference in effect of dry atmosphere, from that of air containing a desirable degree of moisture, is very noticeable in all buildings that are artificially heated. The effect of dry air is made apparent in the average home during the winter months by the shrinking of the woodwork and furniture. The absorption of the moisture from the building which is usually termed “drying out,” causes the joints in the floors and casements to open, doors to shrink until they fail to latch and drawers that have opened with difficulty during the summer then work freely. Winter time is the season of prevalent colds, chaps and roughness of the skin, not so much on account of cold weather as because of dry air. The skin which is normally moist is kept dry by constant evaporation with the attending discomfort of an irritated surface and the results which follow. The humidity of the air in which we live and on which we depend for life has much to do with the bodily comfort we derive in existence, and is suspected of being the cause of many physical ailments. Ventilation engineers not only recognize this condition but have found means of controlling it. It is possible to so control atmosphere temperature and humidity of buildings as to produce any desired condition. =Humidity of the Air.=--The amount of water vapor in the air is called the humidity of the air. It may vary from a fraction of a grain per cubic foot in extremely cold weather, to 20 grains per cubic foot during the occasional hot weather of summer.