IN POPULAR PHYSICS. 59 



69. WJiy is the sheet of zinc under a stove so apt to be- 

 come puckered ? 



When zinc cools after expansion it does not return quite to 

 its former dimensions, and so becomes " puckered," as it is 

 called. 



70. WJiy does a mist gather in the receiver of the air- 

 pump as the air becomes rarefied ? 



"The remaining air, cooled by rarefaction, absorbs heat 

 from the invisible vapor in combination with it, and renders 

 the water visible. The mist may be removed by continued ac- 

 tion of the machine, or by re-admitting the normal quantity of 

 air." 



(See Arnott's Physics, p. 448.) 



71. Why are the tops of high mountains in the tropics 

 covered ivith perpetual snow ? 



See question 59. 



MISCELLANEOUS QUESTIONS AND 

 PROBLEMS FOE REVIEW. 



1. Does a plumb - line point to the earth's center of 

 figure or center of gravity ? 



2. In a dark room, let the light of a candle pass through 

 a small hole in a card, and the image of the candle on the 

 opposite wall will be inverted. Explain. 



3. How many titties heavier is the earth than an equally 

 large globe of water? 



4:. Why does a rocket ascend into the air ? 



5. Is the water at the foot of Niagara Falls warmer 

 than that in the river above ? 



f>. What causes wheel fire-works to rotate ? 



7. A brass rod covered tightly with thin paper may be 

 held some time in a flame ivithout the paper being scorched; 

 while, if the rod be of wood, the paper will scorch at once. 

 Wliy is this difference? 



