To determine the grate area of a vertical boiler, take the diameter of the firebox at the bottom of the firehole and obtain the area. For instance, in a boiler 6 ft. 6 in. high by 3ft. diameter, the firebox at the bottom is 2 ft. 5 1/4. in. At the firebar level, however, this diameter is about l 1/2 in- less, viz. 2ft. 4 in. The area of circle of this diameter = 615.75 sq. in. = 4.27 sq. ft., which is the area of the grate. To obtain the approximate heating surface, multiply the grate area by 10, the ratio of heating surface to grate surface in these boilers being about 10 to 1. Thus, the heating surface in the boiler in question = 4.27 x 10 = 42.7 sq. ft. An approximate rule for the horse-power is to allow 10 sq. ft. of heating surface per horse-power.