Steam and Hot-water Radiators.-The use of steam and hot-water radiators involves the installation of a general heating system of which the radiators are only a small part. As far as efficiency of heating is concerned, there is very little to choose between these two methods. The hot-water system is more expensive to install, but the running expenses are lower than for steam, because it is less expensive to warm the water to 100° C. than to convert it into steam at 100° C. Moreover, in hot-water heating the water is never heated to the boiling-point, hence much less fuel is required.

Heating by Steam.-When water is converted into steam 537 calories are absorbed or rendered latent; I kilogram of water at 100° C. requires as much heat to convert it into steam at 100° C. as would raise 537 kilograms I degree C., or 1 kilogram 537 degrees C. This is termed the latent heat of steam, and in condensing back into water this heat is given off and can be utilized for purposes of heating.

Steam-heating plants are either high or low pressure. High-pressure systems are now generally called expansive systems. They carry steam at a pressure of over 700 grams to the square centimeter, while the low-pressure systems carry steam at less than 700 grams, usually from 150 to 350 grams to the square centimeter. The lowpressure systems are now principally used.

Systems of Piping.-There are three systems of piping in use: (1) The two-pipe system, which is most commonly employed, and can be used for either high- or low-pressure steam. The main return riser is carried below the water-line of the boiler. Various modifica

tions of this system are in use. Each radiator is provided with separate flow and return pipes. (2) A partialcircuit system, in which the main flow pipe rises to the highest part of the basement by one or more branches, whence the distributing pipes run at a slight incline, and finally connect with the boiler below the water-line. The radiators are connected by risers which carry both flow and return from and to the distributing pipes. The

pipes must be made large. This system is employed quite extensively in private houses. (3) The completecircuit system, often called the one-pipe system, in which the main pipe is led directly to the highest part of the building; thence distributing pipes are run to the various return risers, which in turn connect with the radiating surface and discharge in the main return. The supply for the radiating surface is all taken from the return risers, and in some cases the entire downward circulation passes through the radiating system.

Exhaust-steam Heating. This does not imply any particular method of running the pipes, but proper connections must be made between the exhaust and the heating pipes, and provision must be made for taking care of the condensed water.

Hot-water Heating.-On account of its high specific heat water is able to store heat, which it, on cooling, gives up. One kilogram of water in cooling from 100° C. to 20° C. gives up 80 calories, which can heat 8 x 4 32 kilograms of air through 10° C., because the specific heat of air is only one-fourth that of water. Thirty-two kilograms of air are equal to 24.61 cubic meters.

Methods of Piping.-A system of hot-water heating should present a perfect system of circulation from the heater to the radiating surface, and then back to the heater. An expansion tank, on the top floor, must be provided to prevent excessive pressure due to the heating and consequent expansion of the water. In the system ordinarily employed for hot-water heating the mains and distributing pipes have an inclination upward from the heater, while the returns are parallel to the main and have an inclination downward toward the heater, connecting at its lowest part. In this system great care must be taken to produce nearly equal resistance to flow in all the branches leading to the different radiators. It will be found that invariably the principal current of heated water will be in the path of least resistance, and that a small obstruction, as any irregularity in the piping,

is sufficient to make very great differences in the amount of heat received in different parts of the same system.

The expansion tank must in every case be connected to a line of piping which cannot by any possibility be shut off from the boiler. It does not seem to matter whether it is connected with the main flow or with the return.

Combination Systems of Heating.-Several methods have been devised for using the same system of piping alternately for steam or hot water, as the demand for higher or lower temperature might change.

Indirect Heating.-By Means of Steam Radiators.Radiators which are placed in a passage or flue which supplies air to a room supply heat by the indirect method. These heaters are made in various forms. They should be placed in a chamber or box as nearly as possible at the foot of a vertical flue leading to the room to be heated. Air is admitted through a passage from the outside provided with suitable dampers. The chamber surrounding the radiator and the flue leading from the chamber are constructed of masonry or of galvanized iron, and that supplying the cold air of wood lined with tin. There should be a door into the chamber, so that the heater may be cleaned when necessary. It is of great advantage to have a by-pass and mixing dampers in the flues, so that the heated air can be mixed with cold air in order to attain the desired temperature of the incoming air. These dampers are often regulated automatically by means of thermoregulators, whereby the desired temperature is maintained by mixing requisite amounts of heated and cold air.

The system of indirect heating by means of steam radiators in stacks is now very generally in use for large buildings, and when a fan is used to propel the air through the building affords the most satisfactory system of ventilation and heating. When all the arrangements have been properly made the requisite amount of air can be forced into the building and at the desired temperature. In comparison with the efficiency of the ventilation and

heating with this system the cost of the system is no great objection.

Heating with Hot Air.-The general laws which apply to hot-air heating have already been considered in connection with ventilation and the indirect methods of heating. The outside air is conducted through an outer casing surrounding a furnace, and when heated rises through the flues and passes into the rooms above. The rapidity of the circulation depends entirely upon the heat of the furnace and the height of the flue through which it passes. In order that the circulation of air through the rooms may be more perfect, outlet openings must be provided for the escape of the impure air. This system is not adapted for large buildings, because the horizontal distance to which heated air will travel is somewhat limited. When properly proportioned, in buildings of moderate size, this system gives fairly satisfactory results.

In order that the hot-air system may be satisfactory in every respect, the furnace should be sufficiently large, and the ratio of heating surface to grate such that a large quantity of air may be heated to a low degree, rather than a small quantity to a high degree of temperature. The air-supply of the furnace is usually derived from the outside through a shaft specially constructed for this purpose, though in many private dwellings the air is drawn immediately from the basement. The disagreeable effects of the air of furnace-heated rooms are due to the dryness of the air. The principal objection to furnace-heating is the fact that when the supply of heat is shut off, the supply of fresh air is also excluded.

Heating with Electricity.-Electrical energy can be transformed into heat, and as there are certain advantages pertaining to its ready distribution, it is likely to come into more general use for heating. One watt for one hour, which is the ordinary commercial unit for electricity, is equal to 3.41 calories. Electricity is usually sold on the basis of 1000 watt-hours (1 kilowatt) as a unit of measurement, the watts being the product ob

tained by multiplying the amount of current estimated in ampères by the pressure or intensity estimated in volts; on this basis 1000 watt-hours are equivalent to 3410 calories. The expense of electric heating must in every case be very great, unless electricity can be supplied at an exceedingly low price.

Heating by Means of Gas.-In many towns throughout the natural gas region gas is used for heating as well as for illuminating purposes. In these localities gas is the cheapest mode of heating. It is employed in both the direct and indirect systems of heating. When suitable arrangements are made for carrying off the products of combustion, and there is a proper supply of fresh air for purposes of ventilation, this is a very satisfactory method of heating. In cold weather, where the daily fluctuations in the temperature are not very great, the gas heater can be lighted and adjusted, and requires practically no attention for weeks or even months. It is therefore a great saving in time and annoyance, and there is no coal to shovel nor ashes to remove.

Heating by Means of Petroleum.-Within recent years petroleum has been brought into common use for heating purposes. The advantages of oil heaters are that they are portable and may be carried from one room to another, and the amount of heat can be readily controlled. These petroleum stoves are objectionable, however, from the fact that the combustion of the petroleum utilizes large quantities of the oxygen of the air of the room, giving off corresponding amounts of carbon dioxid. Babuke1 has found that in a room of 12 cubic meters capacity, during the first hour the temperature was raised only 4 degrees C., and rose but slowly afterward. The proportion of carbon dioxid in the air exceeded I part per 1000, and reached in the vicinity of the floor 3-10 parts per 1000, and in the upper part of the room 6-12 parts per 1000, amounts which would be detrimental to health when inhaled constantly. The amount of petroleum consumed was about a liter in eight hours.

'Zeitschrift f. Hygiene, Bd. xxxii., S. 33.