WIND. 289 



A. After the bladder is fully infla- 

 ted, (by lying before the fire), if it be 

 taken away from the fire, the bladder 

 will collapse, and show that it is not half 

 full. 



Q. What is meant by the bladder "COLLAP- 

 SING ?" 



A. The skin will become wrinkled, 

 shrivelled, and flabby, because there is 

 not sufficient air inside to fill it out. 



Q. How do you KNOW that CONDENSED air 



Will DESCEND ? 



A. As soon as the cotton of the 

 balloon is burnt out, the air inside becomes 

 cold again, and the balloon falls to the 

 earth. 



Q. Does the SUN HEAT the AIR as it does the 



EARTH? 



A. No ; the air is not heated by the 

 rays of the sun, because air (like water) 

 is a very bad conductor. 



Q. How is the AIR HEATED ? 



A. By convection, thus : The sun 

 heats the earth, and the earth heats the 

 air resting upon it; the air thus heated 

 rises, and is succeeded by other air, which 



ca 



