whole. It is therefore necessary that a furnace which has to eat up fuel at a great pace should be artificially fed with air in the proportion of from 12 to 20 pounds of air for every pound of fuel. There are two methods of creating a violent draught through the furnace. The first is—
The forced draught; very simply exemplified by the ordinary bellows used in every house. On a ship (Fig. 10) the principle is developed as follows:—The boilers are situated in a compartment or compartments having no communication with the outer air, except for the passages down which air is forced by powerful fans at a pressure considerably greater than that of the atmosphere. There is only one "way out"—namely, through the furnace and tubes (or gas-ways) of the boiler, and the funnel. So through these it rushes, raising the fuel to white heat. As