CONSTRUCTION .OF   BLAST   FURNACES.	39
and circumference. There are various plans of coolers used with furnaces. The coolers here illustrated are made of cast-iron about three inches thick by two feet square, and each has three independent coils of one and one-half inch pipe cast in it, so arranged that should the front coil be attacked by the heat as it burns out the lining, it can be shut off, and the inner coils be made operative independently or as a whole. Some furnaces have these coolers made of bronze, cast hollow. It is very seldom trouble is experienced with the coolers shown, and if any should occur arrangements permit their being taken out and replaced. At L is seen a two-inch pipe, perforated with one-eighth inch holes about two inches apart, which encircles the furnace and keeps a constant stream of cool water running* down the plate I which supports the hearth portion of the furnace. This water runs down 011 the outer surface of the plate to a reservoir at R, and which can be filled up with water to a height of about three feet, to protect the lower portion of the hearth with a heavy body of water. A valve is so arranged in the reservoir R that any height of water can be maintain eel in it. It is no unusual occurrence for the metal to break out at this portion of a furnace, resulting in much injury to life and property. The furnace-man's lot is by no means one any need envy, for he shares very fairly the troubles and dangers he has who " meddles with hot iron."