THE HINGES. 97 



frame." Were there not something done to 

 keep these joints oiled, if 1 may so call it, they 

 would not last long. Take the knee, for ex- 

 ample ; and think what a vast deal of friction 

 or rubhing together of the end of the thigh- 

 bone and of the two leg bones, there must be. 



Why, a traveller probably swings each leg, 

 in walking, about 1200 times in a mile. If he 

 should travel 40 miles a day and many travel 

 more than this it would be 48,000 times a 

 day. If he should continue to walk only 30 

 miles a day all the year except Sundays, he 

 would, at the same rate, swing each knee 

 15,024,000 times. 



If he should do this every year, from the 

 time he was 20 years old till he was 70, or for 

 a period of half a century, the number of 

 movements would be 751,200,000 ! 



" A continual dropping," it is said, and it 

 means dropping of water, " will wear away a 

 rock." And the saying, though old, is true. 

 Why, this continued rubbing of the bones of 

 the knee together, if they were allowed to get 

 dry, would wear them so much in a single day, 

 that we should hear a grating noise at every 

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