PRESSURE OF AIR 



117 



of colder air creeping in to take its place 

 the earth are due to this property of air. 

 ency of heated air to 

 rise that makes hot air 

 furnaces useful for heat- 

 ing houses. Valleys 

 are generally colder 

 than the surrounding 

 hillsides, so that deli- 

 cate crops can be grown 

 successfully on the hill- 

 sides although those in 

 the valley are frost 

 bitten. 



The winds of 

 It is this tend- 



55. Pressure of Air. 

 Experiment 62. Use a con- 

 vection apparatus or take a 

 tight chalk box and in two 

 places on the top punch 

 holes in a circle not quite 



HOT Ant FURNACE. 



The hot air rises through the pipes and regis- 

 ters, and cold air presses in from outside. 



as large as the bottom of a lamp chimney. Place 

 a small lighted candle at the center of one of the 

 circles of holes and a lamp chimney, tightly sealed 

 to the box, about each circle. Hold a smoking 

 piece of paper above the chimney which does not 

 inclose the candle. (If a pane of glass is put into 

 one of the vertical sides of the box, better observa- 

 tions can be made.) What happens? Put out the 

 candle and carefully heat the chimney with a 



Bunsen burner. Is there the same action as be- 

 Fig. 53. fore? whv ig it that sparks rise fl . om a fire ? 



What is meant by the draft of a stove ? Why in order to ventilate 

 a room is it best to open a window at the top and bottom? 



Experiment 63. 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 remove from the heat and tightly cork. Be sure not 



