In Fig. 163 is illustrated the form of chimney that is often used for the ventilation of dwellings. This is built with three flues. The flue to the left--marked _A_ at the top--is intended to carry away the smoke and gases from the kitchen range. The flue to the right is that to which is connected the smoke pipe from the furnace. The flue in the middle marked _B_ is for ventilation. Occupying as it does the space between the other two, it is kept warm by the heat of the other flues and the draft is thus increased. Openings to the flue are shown in the different floors at the points _R_ and _S_. The openings are furnished with registers which may be regulated to suit the weather conditions. The dimensions of such a flue may be calculated by the formula given or the area may be taken from the table to correspond with required conditions. In all cases flues should be made ample in size, as they must often do their maximum work under the poorest conditions for the production of good draft. The amount of air discharged from the flue as given in the table is due to the gravitational effect alone. The suction produced by the wind adds in a very large degree to the amount of air discharged. The quantity of air that will flow from a 30-foot flue, by reason of the suction of the wind, blowing 7 miles per hour is equal to the same flue working by gravity with a temperature difference of 20°. With a wind velocity of 7 miles per hour and a temperature as given, the capacity of the flue is doubled. It is easy, therefore, to understand why the rate at which fires burn is so greatly increased by high winds. At the time of very high winds, a chimney flue will carry away three and even four times the volume discharged at the time of atmospheric calm. =Cost of Ventilation.=--The cost of good ventilation is often looked upon as prohibitive, because of the expense in heat necessary to keep the inside atmosphere at standard purity. Cost of ventilation is determined by analysis of the known conditions and calculations made of the amount of extra heat necessary to warm the greater volume of air. The common practice of estimating the quantity of heat used in any form of heating or ventilation is by reference to the B.t.u. used in producing the desired condition. This unit, as has already been stated, is the amount of heat necessary to change a pound of water, 1°F. In considering the cost of heating the air for ventilation, it must be borne in mind that the heat used in raising the temperature of the air contained in an enclosure is only a part of that necessary for warming the building. Most of the heat used goes to keep up the loss due to radiation and conduction which goes on from the windows, the walls and other parts of the building that are exposed to the outside cold. The material of which the building is composed must be heated and in turn radiates its heat to the colder outside air.