2139. The boilers for either of the above modes of heating may be constructed in such a manner as to have the largest possible bottom surface exposed to the action of the fire. According to Mr. Tredgold, for burning a bushel of coals per hour, the area of the firegrate should not be less than 8, or greater than 16, square feet; and the bottom surface should be four times the area of the grate, with 32 feet of side-flue. A considerable advantage, according to that engineer, results from adopting the larger proportion for the grate and bottom surface, on account of the fire requiring less attention. The best mode of regulating the fire is, by means of a door to the ashpit having a register; an improvement which was first suggested by Dr. Black, and afterwards adopted by Count Rumford. One of the most effective boilers is that adopted by Messrs. Cottam and Hallen, Winsley Street, Oxford Street, London (fig. 633.).