114 THE ATMOSPHERE AND ITS SERVICE TO MAN 



as an exit for the stale air, and the upper sash does not have 

 to be lowered. In severe weather this is more successful 

 than the first method. An adjustable ventilator of this 

 kind is shown in Figure 59. 



But the cloth screen is probably the most successful 

 means of steady ventilation in severe weather. For houses 

 that have only casement windows it is about the only method. 

 Make a screen frame that fits snugly into the casement. 

 Cut a piece of muslin to fit the frame and tack it on just as 

 you would wire screening, being sure to stretch the muslin 

 tight as you put it on the frame. With this in place, the 

 casement window may be opened wide in the most severe 

 weather without any danger of direct drafts but with assur- 

 ance of fresh air supply. The cloth screen may be adapted 

 to the sash window, and it is especially useful on stormy 

 nights because it makes it possible to keep a sleeping room 

 window wide open all night. 



Whatever method of ventilation is used, the windows and 

 doors should be opened once or twice every day so that cold 

 fresh air may blow in and flush out the stale air of the rooms. 

 Fresh air and sunlight are man's cheapest doctors. 



Pressure of the Atmosphere. Experiment 44. If a tin can 



with a tightly fitting screw cap can be easily procured, boil a little 

 water in it, having the screw cap open so that the steam can readily 

 escape. While the water is still strongly boiling, quickly re- 

 move the can from the heat and tighten the cap. Be sure not to 

 tighten the cap before removing the can entirely from the heat. 

 Set the tin thus closed upon the desk and observe. What hap- 

 pens as the steam condenses ? Why ? 



Experiment 46. By means of an 

 air pump exhaust the air from a pair of 

 Magdeburg hemispheres. (Figure 60.) 

 Now try to pull the hemispheres apart. FIGURE 60 



