26 SCIENCE OF HOME AND COMMUNITY 



bulb is 70 degrees, the figure opposite 16 and under 70 gives the 

 per cent of humidity, in this case 33. Find the humidity of the 

 air outdoors. 



2. If a sling psychrometer is available, the test can be made 

 more quickly and accurately. 



Comparison of heating systems. We may now examine 

 the various systems of heating with reference to the provision 

 they make for ventilation. No system which fails to make 

 some such provision should receive serious consideration. 



Sfeam or Sfot Water 

 FIG. 5. A method of ventilation in steam and hot-water systems. 



The hot- water and steam heating systems, as ordinarily 

 installed with radiators in each room, are the poorest, as no 

 provision is made for change of air. The hot-air furnace, 

 when fresh air is taken from outside, is preferable as it pro- 

 vides for a change of air. If only one opening is provided, 

 that is, the register where the hot air enters, some 

 ventilation is provided, since some air is forced out around 

 the windows ; but in the best systems, another opening to a 

 flue in the chimney is provided by means of which the used 

 air is taken away. 



