the practical limit of heating air by steam, - together with the weight of vapor, in pounds and percentage, and total heat, with the portion thereof contained in the vapor:

By inspection of above table, it will be seen why it is more economical to dry at the higher temperatures. The atmosphere is seldom saturated with moisture, and in practice it will be found generally necessary to heat the air about 30° above the temperature of saturation. The best effect is produced where there is artificial ventilation, by fan or by chimney, and the course of the heated air is from above downwards.

Hot-Air, Steam, and Hot-Water Heating in Residences.

Much advancement has been made of late years in the methods of heating residences and in the apparatus intended for that purpose. While it is impossible in this book to treat the subject in detail, it is believed that the following information will be of value in deciding upon the kind of heating to be used, and in selecting an efficient apparatus, and seeing that it is properly put in.

In deciding upon a heating apparatus for a dwelling, the governing conditions are, generally, A, the size of the building, and, B, the limit of first cost. When the latter condition is not a control

ling one, the cost of running the apparatus should be given the first consideration.

For residences of eight or ten rooms, and covering not more than 1,200 square feet of ground, the author would recommend hot-air heating by means of a good furnace.

For residences covering 1,400 square feet, a combination hot-air and water system is recommended, or an entire hot-water system. For still larger residences, a steam or hot-water apparatus should be used.

Furnace Heating.—For warming residences not exceeding 1,200 square feet of ground area, the author believes a good furnace, properly set, and with hot-air pipes of proper size, suitably located, will give the best satisfaction, as it is economical in first cost, easy to manage, costs little for repairs, and furnishes a pleasant and healthy heat, at no greater expense of running than with steam or hot water.

The most common defects observed in furnace-heating are: Overheating of the air; vitiating of the air by the gases of combustion; and imperfect distribution of the heat.

The first two defects may be entirely avoided if sufficient care is exercised in the selection and setting-up of the furnace and in tending the fire, and the last defect may be reduced to a minimum by a wise location and proper proportion of the flues and registers.

The cause of the unsatisfactory heating of a great many houses, by furnaces, is in the owner or builder refusing to pay the necessary price for a first class furnace and for the best workmanship and materials. The same carelessness and "skinning" that is sometimes permitted with furnace-work, if permitted on a steam or hotwater apparatus, would in most cases prevent their working at all. Furnace heating may be divided into two parts, the production of heat, and the distribution of the heat.

The former depends entirely upon the furnace, its setting, coldair supply, draught, kind of fuel, and attendance.

The Furnace.-In principle, a hot-air furnace is simply a stove or heater, encased with iron or brick, so as to form an air chamber between the heater and casing. The air enters at the bottom of the chamber, passes over the heated surfaces of the heater, and is conducted by the hot-air pipes to the various rooms.

The external surface of the fire-pot, and all portions of the heater which receive heat from the fire or smoke, are called the radiating surface.

As a rule, the furnace which has the greatest radiating surface in proportion to the size of the fire-pot will give off the most heat for a given amount of fuel consumed.

As the amount of radiating surface largely affects the weight of a furnace, and the latter in a great measure the selling price, it is obvious that the best furnaces must cost the most. It is true that one furnace may have its radiating surfaces better arranged than another, so as to give off more heat for a less quantity of metal, but it is seldom that a very light furnace, particularly if of cast iron, is a good heater.

Furnaces should be so designed that the smoke, after leaving the combustion chamber, must travel around the radiator one or more times before finding an exit to the chimney. With a chimney flue of proper size and topped out well above the roof, it is possible to make the smoke travel a long distance, and thus obtain great economy of fuel. The best furnaces are designed on this principle.

Besides having large radiating surface, the furnace should have as few joints as possible, and should be arranged so as to be easily cleaned.

Furnaces are made of cast iron, wrought iron, and steel, either used singly or combined. The radiating surface above the fire-pot can be made more cheaply of wrought iron than of cast iron, and in certain arrangements it is just as serviceable.

While there are excellent furnaces made of wrought iron and steel, the author believes that a heavy cast-iron furnace is the most durable, and can be made as tight. Some furnaces are made chiefly of cast iron, but with air or smoke flues of wrought iron fitting into cast-iron sockets. This arrangement is not generally approved, as the two metals expand and contract unequally, thus tending to open the joint.

There are so many styles of furnaces manufactured that it is quite impossible to go further into details. It may be said, however, that the furnace shown in Fig. 1, made by the Richardson &

Boynton Company, is representative of the best type of cast-iron furnace, and that shown in Fig. 2, made by Isaac A. Sheppard & Co., a modern steel-plate furnace. Fig. 3, of which the Excelsior Steel Furnace Company are the makers, shows a type of furnace which consists of a plain combustion chamber with a steel radiator. This radiator is divided with a horizontal partition, so that smoke must circulate entirely around it before it enters the flue. This furnace is intended for soft coal. The more modern furnaces, constructed for burning

soft coal, have provision for the introduction of superheated air into the fire-box, thereby preventing the formation of soot, and causing thorough combustion and intense heat. The one shown in Fig. 1 is a hot-air blast furnace, and is supplied with oxygen at a high temperature for either hard or soft coal, accelerating and intensifying combustion to a very high degree. The Thatcher Furnace Company are makers of a tubular furnace that seems to possess considerable merit

The casing surrounding the heater may be of brick or sheet iron. If of brick, it should consist of two four-inch walls with a space

between, the inner wall being generally built on a circle, and the outer one on a square.

Brick set" furnaces are not as common as they formerly were, as they can be cased as well with iron, and without occupying so much space in the cellar. When cased with sheet iron, the furnace is designated as "portable." Portable furnaces should always have a double casing with an inch space between. The inner casing may be of black iron, but the outer one should be galvanized. The hot air is thrown into the pipes better if the top of the casing is truncated, as in Fig. 2.

Cold Air Supply.-In a house heated by a furnace, the temperature of the rooms is maintained by a constant incoming current of hot air, and it is absolutely necessary for satisfactory heating that proper provision be made for supplying this air to the furnace, and on no account should a hot-air furnace be used without being provided with a direct supply of air from outside the building. In dwellings, this may be best accomplished by putting an opening in the external wall just beneath the first floor joist, and as far above the ground as the elevation of the building will permit. From this opening, which should be covered with galvanized wire netting of about three-eighths of an inch mesh, a duct or flue should be carried to the air-pit under the furnace, as shown in Fig. 3.