may be reduced one-half at the point of exit by doubling the capacity of the register box and the pipe for a short distance from the outlet. In designating registers, the sizes are taken from the outside of the register face. The clear opening is about two-thirds the register face. AREA OF ROUND REGISTERS. The following table shows the sizes of registers and leader pipes commonly employed in ordinary dwellings: SIZE OF HOT-AIR PIPES AND REGISTERS. 1e heating engineers proportion the registers to the contents of the room to be warmed; the free area of ,ister, in square inches, per 1,000 cu. ft. of space for different velocities of air discharge from the registers and for different numbers of air changes per hour are given in the accompanying table. REGISTER PROPORTIONS IN REFERENCE TO Velocity, in Feet per Minute. 120 180 240 300 360 480 600 900 1,200 Net Free Area of Register, in Square Inches, for 40 20 80 40 120 60 160 80 200 100 66.0 240 120 80.0 60 320 160 107.0 80 400 480 16.0 12 21.6 27.0 Pipes that serve to convey steam from the boiler, or other source of supply, and distribute it to several branches are known as steam mains. They are usually run along the cellar ceiling, being hung from the first-floor beams by adjustable iron hangers. They pitch downwards from the highest point near the boiler to the lowest point at the farther end of the mains. The pitch should be at least in. in 10 ft., so that the water of condensation may freely flow to the lower end of the main. An overhead main is a steam main that is run horizontally, or nearly so, at an elevation higher than the radiators that it supplies. Risers are vertical pipes that rise from floor to floor to convey steam from the steam main to the radiators or coils on the several floors. Drop risers are those in which the steam flows downwards to the radiators or coils from a steam main above, usually in the attic. A return main is a nearly horizontal line of pipe, usually run near or under the cellar floor; it receives all water of condensation from the heating system and returns it to the boiler, or otherwise disposes of it. A dry return main is one that is run above the water-line of the boiler and, consequently, is partly filled with steam. A wet return main is one that is run below the water-line and is filled with water at all times. As a rule, this is more reliable than a dry return main, except in places where the main is subject to frost. Return risers are those vertical pipes that receive the water of condensation from the radiators or coils on the several floors of a building and convey it to the return main. A drip pipe, relief, or bleeder, is a small pipe used to drain water of condensation from the foot of the risers or from a low point, pocket, or trap in the main steam pipes. Riser connections are the pipes, usually short and nearly horizontal, that connect the steam main to the lower ends of the risers or that connect an overhead main to the upper ends of drop risers. Radiator connections are the pipes that connect the radiators to the risers or mains; they are usually short and seldom larger than 2-in. pipe. Direct radiation is a term applied to all kinds of coils and radiators placed inside the rooms to be heated. Indirect radiation is a term applied to all kinds of coils, radiators, and other forms of heating surfaces located outside the rooms to be warmed. Indirect radiators are usually hung from the cellar ceiling, are encased with a galvanized sheet-iron jacket, and are so constructed that fresh air from the outer atmosphere flows between the heating surfaces and enters the room, thus providing ventilation as well as heat. Direct-indirect radiation, sometimes called semidirect, is a term applied to radiators and coils located in the rooms to be warmed and provided with connections to the outer atmosphere, so that fresh air may enter the rooms through the radiator flues and be warmed thereby. METHODS OF HEATING BY STEAM. The various systems of heating by steam may be classed in a general way as (1) high-pressure systems; (2) low-pressure systems; (3) vacuum, or exhaust, systems. In the first class are all systems of heating operated at a pressure greater than 10 lb. by the gauge; in the second class are those operated at pressures between that of the atmosphere and 10 lb. by the gauge; in the third class are all systems operated at a pressure lower than that of the atmosphere. Any one of these systems may be subdivided as follows: (1) the one-pipe system; (2) the two-pipe system; (3) the two-pipe system with separate risers; (4) the overhead-main or drop-supply system. These, in turn, may be gravityreturn systems or forced-return systems, and they may have wet-return or dry-return mains. In the gravity-return system, the water of condensation flows back to the boiler by gravity. This is used in cases where the full boiler pressure is allowed on the heating system. It cannot be used elsewhere. The forced-return system is that in which the water of condensation is forced back to the boiler from the return mains of the heating system by a pump, steam loop, steam-return trap, or other such contrivance. This system is used when the boiler pressure is higher than that in the heating system, as, for example, when a pressure-reducing valve is used on the steam-supply pipe to the heating system. A common arrangement of the one-pipe system of heating is shown in Fig. 1. The boiler a furnishes steam to the steam main b, by which it is conveyed to the several risers c, c, thence flowing to the radiators d, d, etc. within the rooms to be warmed. The main b is so suspended from the floor joists by hangers as to have a uniform fall from its highest point, which is immediately above the boiler, to its lowest point f. A pitch of about in.'in 10 ft. is usually considered a sufficient fall for the main. When steam is generated in the boiler, it is forced into the steam main, from there into the risers, and thence into the radiators. The air that the pipes contain is forced out of the system to the atmosphere through air vents placed at suitable points in the system, usually on each |