56 ANSWERS TO PRACTICAL QUESTIONS 



46. Which can be ignited the more easily 'with a burn- 

 ing-glass, black or white paper ? 



Black paper, since it is a much better absorber of heat. 



47. Why does the air feel colder on a windy day ? 



Because fresh portions of cold air are brought constantly in 

 contact with our bodies. 



48. Could a burning-lens be made of ice ? 



Burning-lenses have been made of that material. The rays 

 have no heating power until the waves of ether are stopped. 

 They do not elevate the temperature of the medium through, 

 which they pass. 



49. IVhy is an iceberg frequently enveloped by a fog ? 



The moisture of the air is condensed upon its cold surface. 



50. Would dew gather more freely on a rusty stove 

 than on a bright Kettle ? 



It would, because the rusty iron surface is a good radiator. 



51. Why is a clear night colder than a cloudy one dur- 

 ing the same season ? 



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. 



52. Why is no dew formed on cloudy nights ? 



See last question. 



53. Why will "fanning " cool the face ? 



It brings in contact with the face a current of fresh and 

 generally cooler air. 



54. How are safes made fire-proof? 



By filling the space between the inner and the outer iron 

 plates with a non-conducting material, as plaster, etc., the safe 

 is rendered nearly fire-proof. 



