HEATING 493 HEATING regqfarly reflected in all directions, it may become scattered or diffused heat; reflection and refraction may also polar- ize its rays, as happens to those of solar light. The heat which falls on a body is called incident heat. Specific heat is the quantity of heat required to raise the temperature of a body of a given weight 1°; the unit of measure being the quantity required to raise the same weight of water to the same tempera- ture. Heat may be produced by solar radiation, chemical action, friction, pres- sure, percussion, absorption, and imbibi- tion; by the conduction of powerful magnets and bodies in motion, etc. HEATING and VENTILATION. In cold climates, artificial heating and ven- tilation are both necessary for health and comfort and are equally important. Because the discomfort produced by low temperature is more acutely and im- mediately felt than that due to foul air, improper ventilation is more common than deficient heating. The two subjects are so closely related, however, that they will be considered together in this article, the question of heating being first dealt with in greater detail. The temperature most conducive to bodily comfort cannot be stated definite- ly as it is largely determined by climate and personal habit, varying in different countries. A temperature of 68-70° F. is considered necessary in the U. S. A. and Canada, but 62° F. is the tempera- ture most favored in England. All sys- tems of heating depend upon radiation or convection; or, more generally, upon a combination of the two. The common- est example of radiation is the open fireplace which radiates heat, and so warms the walls and furniture of the room, while leaving the air compar- atively cool. An example of convection is found in the hot air furnace, which supplies currents of warm air, which constantly replace the cold air. Radia- tion and convection combined are found in the ordinary steam or hot water radi- ator, which heats the air by convection and also radiates some heat to sur- rounding objects. Heat is most com- monly produced by the combustion of coal, wood, coke, oil, gas, or some other fuel, but is occasionally obtained elec- trically, the electricity being produced by water-power. Comparing the relative merits of the different forms of heating the open fire- place is popular because it is cheerful to see, and from the hygienic point of view it is good because it produces a simple but efficient means of ventilation. It utilizes, however, only 10-15 per cent. Vol. IV — Cyc — FF of the heating value of the fuel, and, unless supplemented by other forms of heating, is quite inadequate for pro- viding sufficient heat in cold climates. The stove which stands out in the room, being connected to the chimney by a pipe, is upward of 50 per cent, efficient, but it has the disadvantage of being dusty, and of quickly producing foul dry air unless careful attention is given to ventilation. The hot-air furnace is of two types. The older type conveys the heated air from a central chamber to various parts of the house by means of pipes; the more modern type is pipeless, the heat entering the upper part of the house from a single register on the first floor, and being carried by convection to all parts of the house. The furnace is supplied with fresh air from the out- side, and provided this feature is prop- erly cared for, a hot air system is prob- ably the most healthful method of heat- ing a dwelling. With the old type of furnace, however, it is difficult to obtain uniform heating, the rooms on the wind- ward side of the house being cold, while those on the sheltered side are over- heated. The pipeless furnace is growing in popularity, and gives satisfaction in houses of suitable size and design. The steam furnace generates steam from a boiler in the cellar and distributes it over the house by means of pipes con- nected to radiators. The hot-water fur- nace is similar, except that hot water instead of steam circulates through the pipes and radiators. Both systems have advantages and disadvantages. Hot wa- ter is more difficult and expensive to install but has the advantage that it begins to supply warmth as soon as the water becomes heated, whereas, with steam, the water must boil before heat is supplied. Hot water is also quieter than steam, but high temperatures can be produced more rapidly with the latter and much less radiating surface is needed. A modern development of steam heat is the so-called "vacuum system," in which the whole system of piping and radiators is maintained under a slight vacuum. One advantage of this system is that the knocking and hissing of the radiators is avoided. Another recent modification is the gas-steam radiator, which is a radiator having a small res- ervoir of water at the base, heated by gas burners. .A.S the pressure, due to gen- eration of steam, rises, the gas is auto- matically lowered. Electric heating is too expensive for use on a large scale, but finds application in small heaters for intermittent use, and also in the heating of street cars. Ventilatiim.—A steady supply of fresh
