14 ANSWERS TO PRACTICAL QUESTIONS 



IS. In casting large cannon, the gun is cooled by a 

 stream of cold water. Why ? 



The object of this is to cause the iron to cool more quickly, 

 and so not give the molecules time to arrange themselves in 

 crystals. 



19. Why does paint adhere to wood? Chalk to the 

 blackboard ? 



These are illustrations of the force of adhesion. 



20. Wliy does a toivel dry one's face after washing ? 



The sensible pores of the cloth absorb the water from the 

 face by adhesion. 



21. Why will a greased needle float on water ? 



The grease prevents the needle from being wetted, and the 

 toughness of the surface film cf water is sufficient to with- 

 stand the weight of the needle. 



22. IVhy is the point of a pen slit ? 



So that we may widen at will the surface of contact be- 

 tween the ink and the paper. The ink is prevented from de- 

 scending rapidly, when the slit is not open, by the grip of its 

 surface film. 



23. Why is a thin layer of glue stronger than a thick 

 one ? 



The adhesion between the glue and the wood is stronger 

 than the cohesion between the particles of glue ; hence the 

 thinner the layer of glue the fewer the particles acted upon 

 only by the latter or weaker force. 



GRAVITATION. 



73 ! JVlien an apple falls to the ground, hoiv much 

 does the earth rise to meet it ? 



The earth falls as much less distance than the apple, as its 

 mass is greater. 



