384 SECTION MECHANICS. 



ton of small coal to twenty tons of ore ; and in calcining in open heaps, 

 in South Wales and Staffordshire, it is given as one ton of small coal, 

 and five hundredweight of large, to ten tons of ore.* 



A similar economy in fuel to that which characterizes such continu- 

 ous kilns is attained, even more perfectly, in calcining limestone 

 or other materials, and more especially in burning bricks, by the system 

 of annular kilns patented some years ago by F. Hoffmann of Berlin, 

 and now very generally used. In this form of kiln, which is continuous 

 in its working, the air to supply the fire is drawn through the bricks 

 already burned, cooling them down to the temperature of the air, and 

 carrying forward their heat to the part where it is required ; and the 

 burned gases, mixed with excess of air, pass, on their way to the 

 chimney, through the stacks of damp air-dried bricks, that have not 

 yet been fired ; and escape, finally, at a comparatively low temperature, 

 and saturated with moisture. 



The kiln is built in the form of an arched passage, eight or nine feet 

 high, and of the same width. This tunnel or passage is bent on itself, 

 in the form of a ring of any convenient diameter, and provided with 

 twelve equidistant doorways for putting in and removing the bricks, 

 and twelve corresponding flues, leading to a central chimney, and each 

 provided with a damper. The entire kiln thus consists of twelve 

 or more chambers or compartments, arranged in a circle and com- 

 municating freely with each other; each compartment having an 

 independent entrance door and chimney flue. 



The successive compartments being numbered one to twelve, the 

 working of the kiln is as follows : Supposing No. 2 to be the com- 

 partment from which the burned bricks are being removed, and No. i 

 to be the adjoining compartment, last emptied, and now being filled 

 with new unburned bricks ; then No. 12, on the other side of No. I, 

 will be the compartment last filled with new bricks, which are now 

 being dried preparatory to burning. When the work is in this posi- 

 tion, the damper in the flue leading from compartment No. 12 to the 

 chimney, and tha entrance doorways of Nos. I and 2 are open, all the 

 other chimney flues and the other door openings being closed ; and 



* Metallurgy, J. A. Phillips, pp. 181, 189. 



