The general rule is to make the cross-section of the chimney, which may be either round or square, from 1/6 to 1/10 of the grate-surface, and the height from 50 to 70 feet.

To determine the amount of coal which will be burned per square foot of grate per hour, with good proportions, by Professor Thurston's rule.-Subtract one from twice the square root of the height. Example: What will be the amount of coal burned per square foot of grate surface per hour, the chimney being 49 feet high, and suitably proportioned? The square root of 49 is 7; twice 7 is 14; 14 less 1 is 13, which is the amount of coal required in pounds.

To determine the height required to give a certain rate of combustion.-Add 1 to the weight to be burned per square foot per hour; divide by 2 and square the quotient. Example, same as above, worked backwards, thus: What height of chimney is required to burn 13 pounds of coal per square foot of grate surface per hour? 13 and 1 are 14; 14 divided by 2 is 7; the square of 7 is 49, which is the height of the chimney in feet.