It has been found that air cooling is not always satisfactory for the reason that when the air is cooled by means of an air washer, the relative humidity of the air is increased, and air saturated or nearly saturated with moisture is not as pleasant as air at a some what higher temperature where the relative humidity is lower. To overcome this objection air cooling plants have been installed where i the cooling is carried to such an extent as to lower the temperature of the air considerably below that desired to maintain in a room, and to such an extent to condense part of the mois ture that air contains in order that heat may be added to the air after cooling so as to obtain, finally, air at a higher temperature than the temperature of air leaving the washer, but with a lower relative humidity. The net result of this operation is to admit to the room or building air at a lower temperature than the outside air and of a relative humidity that is comfortable. The first cost of apparatus neces sary to do this and the cost of operation is considerable, and the expense involved is only justified under special conditions.
Heating by electricity is not done to any great extent, on account of the excessive cost. When coal is burned under a steam-boiler, it is not uncommon for 60 per cent of the heat in the fuel to be realized in the steam which can be used for heating. If the heat in coal be transformed into electrical energy, and this again transformed into heat, less than 10 per cent of the heat in the fuel will be realized for heating.
Bibliography.—Tredgold, 'The Principles of Warming and Ventilating Public Buildings, Dwelling Houses, etc.); Hood, 'A Practical Treatise on Warming by Hot Water); Peclet, 'Traite de la Chaleur) • Briggs (and Wolff), 'American Practice in Warming Buildings by Steam' ; Billings, 'The Principles of Ventila tion and Heating); Mills, (Heat—Its Applica tion to the Warming and Ventilation of Build ings); Baldwin, (Steam-Heating' ; 'Hot-Water Heating and Fitting); Monroe, (Steam-Heat ing and • Carpenter, 'Heating and Ventilation of Buildings.'