46 ANSWERS TO PRACTICAL QUESTIONS 



On a cloudy night the clouds reflect the radiated heat of the 

 earth back again, and thus act as a blanket to keep the earth 

 warm. On such a night there can be no frost or dew. On a 

 clear night, the heat which the earth radiates passes out freely 

 into space, and thus the earth cools rapidly. 



54. Why is no dew formed on cloudy nights f 

 See last question. 



55. Water boiled at a certain place at 200 F. : what wa* 

 the height above the sea f 



56. On Mont Blanc boiling water is only 84 C. : what is 

 the height? 



57. Why do we use a longer tube of mercury for a barometer 

 than a thermometer ? 



58. Which is the hottest part of a room ? 



59. Why is it hotter above a flame than at the side f 



60. What is the difference between dew and rain f 



6 1 . Why will ashes keep fire overnight ? 



62. If a pane of glass and a similar plate of polished steel 

 were laid upon the ground, in the night, upon which would 

 the dew form most abundantly ? 



The glass is a poor conductor of heat, and so would absorb 

 little heat from the earth, while the metal would absorb it 

 freely ; the glass is a better radiator than the polished metal, 

 and thus would become drenched with dew, while the metallic 

 surface would be scarcely dimmed. 



63. Why is there but little dew formed in cities ? 



64. Is an abundant dew a sign of rain f 

 It is. See question 32. 



65. Is there any dew formed out at sea ? 



66. Why are gardens in a valley often touched with frost, 

 while those on the hills escape unharrf.ed / 



The cold air settles into the valley ; while the warm air rises 

 to the hills. 



