It is evident that the temperature of the escaping gases cannot be brought below that of the absorbing surfaces, while it may be much greater even to that of the fire. This is supposing that all of the escaping gases have passed through the fire. In case air is allowed to leak into the flues, and mingle with the gases after they have left the heating surfaces, the temperature may be brought down to almost any point above that of the atmosphere, but without any reduction in the amount of heat wasted. It is in this way that those low chimney temperatures are sometimes attained which pass for proof of economy with the unobserving. All surplus air admitted to the fire, or to the gases before they leave the heating surfaces, increases the losses. We are now prepared to see why and how the temperature and the rapidity of combustion in the boiler furnace affect the economy, and that though the amount of heat developed may be the same, the heat available for the generation of steam may be much less with one rate or temperature of combustion than another. Assuming that there is no air passing up the chimney other than that which has passed through the fire, the higher the temperature of the fire and the lower that of the escaping gases the better the economy, for the losses by the chimney gases will bear the same proportion to the heat generated by the combustion as the temperature of those gases bears to the temperature of the fire. That is to say, if the temperature of the fire is 2500 degrees and that of the chimney gases 500 degrees above that of the atmosphere, the loss by the chimney will be 500/2500 = 20 per cent. Therefore, as the escaping gases cannot be brought below the temperature of the absorbing surface, which is practically a fixed quantity, the temperature of the fire must be high in order to secure good economy. The losses by radiation being practically proportioned to the time occupied, the more coal burned in a given furnace in a given time, the less will be the proportionate loss from that cause. It therefore follows that we should burn our coal rapidly and at a high temperature to secure the best available economy. [Illustration: Portion of 9880 Horse-power Installation of Babcock & Wilcox Boilers and Superheaters, Equipped with Babcock & Wilcox Chain Grate Stokers at the South Side Elevated Ry. Co., Chicago, Ill.] PROPERTIES OF WATER Pure water is a chemical compound of one volume of oxygen and two volumes of hydrogen, its chemical symbol being H_{2}O. The weight of water depends upon its temperature. Its weight at four temperatures, much used in physical calculations, is given in Table 10. TABLE 10 WEIGHT OF WATER AT TEMPERATURES USED IN PHYSICAL CALCULATIONS