Some Applications of Heat 113 



hot water pipes are installed underneath the floor. They are 

 enclosed in boxes which are connected with the outside by 

 means of long flues through which fresh air enters. As it 

 flows into the boxes and comes in contact with the heated coils, 

 the air is warmed and rises through grated openings into the 

 rooms. 



In the second method, the direct-indirect system, the radiators 

 in the rooms to be heated are usually placed near the windows. 

 The fresh air enters the t building from the outside through an 

 opening in the wall and is conveyed to openings in the floor under 

 the radiators in each room. As air passes over the heated radia- 

 tor, it becomes warm and carries the heat by convection currents 

 around the room. 



When the air is blown into a building by force, the method 

 is known as the plenum system, and when it is drawn in by suc- 

 tion it is known as the vacuum system. In the plenum system the 

 fan is generally installed in the basement. In the vacuum system, 

 it is commonly installed in the attic. By the plenum system the 

 air is forced over the heated surfaces of coils of pipe and 

 then through air passages into the rooms. By the vacuum 

 system the fan draws out the foul air, and fresh air is pushed 

 into the air passages by the pressure of the outside atmosphere. 

 Sometimes the two systems are combined in order to make ven- 

 tilation more effective. 



In buildings where there is no system of ventilation, fresh air 

 may be secured and foul air gotten rid of by merely opening the 

 windows at the top and bottom. By this method convection cur- 

 rents are set up which are quite effective in changing the air of 

 a room. 



Experiment to Show, by the Use of a Model, a Good 

 Method of Ventilating a Room. Prepare a box about two feet 

 long, eighteen inches high and six inches wide, with a sliding glass 

 front. To represent windows, bore four ^-inch holes in each 

 end, two in the upper and two in the lower part. Fit a cork in each 

 opening. In the box, about six inches from each end, set two 

 lighted candles. Close the glass front. Then (a) open the holes 

 in the lower part of one end and in the upper part of the other 



