CONDUCTION AND RADIATION 93 



35. Which is best for an early garden, a level field or one sloping toward 

 the south? Why? 



36. Why does a black object melt into snow or ice faster than a 

 lighter-colored one? 



37. In regions where there is much sunlight, water for domestic pur- 

 poses is sometimes heated by being passed through a coil of pipes exposed 

 to the sun's rays. The pipes are painted black. Why? 



38. The pipes mentioned above are usually enclosed in a box with the 

 side toward the sun made of glass. How does this increase its efficiency? 



39. Would it be of any use to surround the box with a layer of straw, 

 excelsior or sawdust? Explain. 



40. How does spreading cloth or paper over plants protect them 

 from the frosts of early autumn? 



41. When dew gathers on plants, how does it prevent further radiation 

 of their heat (76)? 



42. Why does no dew form on cloudy nights? (A lowering of the tem- 

 perature is necessary for the formation of dew.) (82.) 



43. Why does the floor feel colder to the bare feet than a rug or carpet? 



44. The Sahara desert is in the tropics and yet the air there may become 

 so cold at night as to freeze water. Why? 



46. Why do we receive less heat and light from the sun at morning and 

 evening than at midday? 



46. In winter the sun is some millions of miles nearer the earth than 

 it is in summer and yet it is hotter in summer. Explain. 



