IN POPULAR PHYSICS. 51 



2. Why does a piece of bread toasting curl up on the 

 side toward the fire ? 



The v/ater being expelled from the pores on that side causes 



tho bread to shrink. 



3. Wluj do double ivindoivs protect from the cold ? 



The non-conducting air inclosed between the window-panes 

 keeps in the heat and keeps out the cold. 



4. Why do furnace-men wear flannel shirts in summer 

 to keep cool, and in ivinter to keep warm, ? 



In summer the non-conducting flannel keeps out the fur- 

 nace-heat, and in the winter keeps in the body-heat. 



5. Why do ^ve blow our hands to make them warm, 

 and out- soup to make it cool ? 



Our breath is warmer than our hands, but cooler than our 

 soup. 



6. Why does snow protect the grass in winter ? 



The air inclosed between the flakes of snow is a non-con- 

 ductor. No infant in its cradle is tucked in more tenderly than 

 the coverlet of snow about the humble grass that nestles down 

 for its winter's nap on the bosom of Mother Earth. 



7. Why does water "boil away" more rapidly on 

 some days than on others? 



Because the atmospheric pressure varies, independently of 

 the fact that the source of heat may vary without our noticing it. 



8. What causes the crackling sound in a stove ^vhen a 



fire is lighted ? 



Tho expansion of the iron by the heat. 



,9. TTh ?//.? the tone of a piano higher in a cold room 

 than in a warm one? 



The steel wires lengthen in a warm room, and so lower the 

 tone. 



