RULE.--Add the area of the glass surface in the room to one-quarter of the exposed wall surface, and to this add from one-fifty-fifth to three-fifty-fifths of the cubical contents (one-fifty-fifth for rooms on upper floor, two-fifty-fifths for rooms on first floor and three-fifty-fifths for large halls); then for steam multiply by 0.25, and for hot water by 0.40. _Example._--A room 20 by 12 by 10 feet with glass exposure of 48 feet, 1/4 of wall exposure (two sides exposed) 320 feet = 80, 1/55 of 2400 = 44. 48 + 80 + 44 = 172 × 0.25 = 43 feet. If you add 2/55 the surface would be 54 feet. If you add 3/55 the surface would be 65 feet. PROPORTIONING THE SIZE OF MAINS For any size system of steam or water heating the following rule will be found entirely satisfactory for mains 100 feet long; for each 100 feet additional use a size larger ratio. RULE.-- _r_ = (3.1416/_d_)_R_ = _a_/_r_ × 100. _r_ represents ratio of main in inches for each 100 feet of surface; _d_, diameter of pipe; _R_, quantity of radiation carried by size of pipe; _a_, area of pipe in inches. From this the following table has been constructed: ---------+---------+----------+----------------- Diameter | Area | Ratio to | Quantity of of pipe | of pipe | each 100 | radiation, steam | | feet of | or water, on a | | surface | given size pipe ---------+---------+----------+----------------- 1½ | 1.767 | 2.10 | 84 2 | 3.141 | 1.57 | 200 2½ | 4.908 | 1.25 | 400 3 | 7.069 | 1.04 | 700 3½ | 9.621 | 0.90 | 1,062 4 | 12.566 | 0.78 | 1,590 4½ | 15.904 | 0.70 | 2,272 5 | 19.625 | 0.63 | 3,120 6 | 28.274 | 0.52 | 5,440 7 | 38.484 | 0.45 | 8,550 8 | 50.265 | 0.40 | 12,556 9 | 63.617 | 0.35 | 18,100 10 | 78.540 | 0.30 | 25,300 ---------+---------+----------+----------------- FORMS OF RADIATORS Radiators are much the same in appearance for both steam and hot-water heating. They are hollow cast-iron columns so designed that they may be fastened together in units of any number of sections. The sections are made in size to present a definite number of square feet of outside surface that is spoken of as radiating surface. The amount of radiating surface in any radiator depends on its height and the contour of the cross-section. The radiator sections may be made in the form of a single column as Fig. 22 or they may be divided into two, three, four or more columns to increase their radiating surface.