Molecular Attraction 55 



it is to pull a wagon forward than it would be to take hold 

 of the wheels and pull the wagon sidewise. So when 

 you want the least possible friction in a machine you use 

 ball bearings. The bearings are located in the hub of 

 a wheel. Then, instead of the axle rubbing against the 

 hub, the bearings roll inside of the hub. This causes 

 very little friction ; and the friction is made still less by 

 keeping the bearings oiled. 



Application 14. Suppose you were making a bicycle, 

 in which of the following places would you want to increase 

 the friction, and in which would you want to decrease it? 

 Handle grips, axles, pedals, tires, pedal cranks, the sockets 

 in which the handle bar turns, the nuts that hold the parts 

 together. 



Application 15. A small boy decided to surprise his 

 mother by oiling her sewing-machine. He put oil in the 

 following places : 



On the treadle, on the large wheel over which the belt 

 runs, on the axle of the same wheel, on the groove in the 

 little wheel up above where the belt runs, on the joint where 

 the needle runs up and down, on the little rough place under 

 the needle that pushes the cloth forward. Which of these 

 did he do well to oil and which should he have let alone? 



Inference Exercise 

 Explain the following : 



61. Rivers flow north as well as south, although we usually speak 



of north as "up north." 



62. Tartar and bits of food stick to your teeth. 



63. Brushing your teeth with tooth powder cleans them. 



64. When a chair has gliders (smooth metal caps) on its feet, it 



slides easily across the floor. 



65. When you wet your finger, you can turn a page more easily. 



66. A lamp wick draws oil up from the lower part of a lamp to the 



burner. 



