2120. The boilers used to generate steam are formed of cast or wrought iron, or copper, and are of different shapes. Wrought iron, or copper, presenting a concave bottom to the fire, are generally preferred at present. Mr. Stothert of Bath, who has had extensive experience in heating by steam, has given the following as the principles on which he calculates the size of boilers: ï¿½ 1. For forcing-houses under ordinary circumstances, he finds that 1 square foot of bottom surface of boiler is equivalent to 150 square feet of glass. 2. Where bottom heat is required, 1 square foot to 135 feet of glass. 3. For greenhouses, 1 square foot is equal to 200 feet. For houses that are ill glazed, or placed in an exposed situation, ten or fifteen per cent. may be added to the proportionate surface of the boiler.