2107. As to the size of hothouse fireplaces, the door of the furnace may be from 10 in. to 1 ft. square; the fuel-chamber from 2 to 4ft. long, from 18 in. to 2ft. wide, and of the same dimensions as to height. Every thing depends on the kind of fuel to be used. For Newcastle coal, a chamber 2 ft. long, 18 in. broad, and 18 in. high, will answer as well as one of double the size, where smoky Welsh or Lancashire coal is to be used. Various contrivances, as hoppers, horizontal wheels, &c., have been invented for supplying fuel to furnace-fires without manual labour, and especially during night; but, from the nature of combustion, and the common materials used in this country to supply it, no effectual substitute has yet been discovered. If wood or charcoal, or even cinders or coke, were used, there would be a greater chance of such inventions succeeding; but we do not think ourselves warranted in detailing any of them.