IRON MANUFACTURE 393 correspond to the rate of the shrinkage, which is more rapid as the charge approaches the tuyeres. This principle is generally recognized ; but it is applied in practice to the lower part of the furnace only. It seems rational to sup- pose that advantage would result by applying it to the whole length of the furnace, and making the greatest diameter correspond to that part where the charge occupies the great- est bulk, namely, at the mouth. The difficulty of properly distributing the charges over a wide mouth is however at present a practical objection ; and it may be also that the reac- tions in the upper zone of the furnace, by which carbon is deposited, would be unfavorably affected by such a construction. The height Fio. I. of the furnace depends primarily on the nature of the charge. If this is disposed to crumble, or is composed of fine particles that might Eack and impede the passage of the blast, a igh furnace would be inadmissible ; but, other things being equal, the higher the furnace the greater is its yield and economy of working, as the reducing gases are more thoroughly in- tercepted and utilized. Fig. 1 is a vertical sec- tion through the fore hearth of a German blast furnace built entirely of masonry. Its height is 48 ft. and greatest diameter 14 ft. Fig. 2 is a vertical section of a blast furnace at Chicago. It is 66 ft. high and 17 ft. in greatest diameter. The top is closed by a " bell and hopper." The upright column at the side is the gas conductor. Fig. 3 is an elevation of the same furnace show- Fio. 8.
