56 



GRAVITATION. 



of the base. A porter carrying a pack is obliged to lean forward ; 

 a man carrying a load in one hand is obliged to lean away from the 

 load, to keep the common centre of gravity of man and load over 

 the base formed by joining the extremities of his feet. Why does a 

 person stand less firmly when his feet are parallel and close together 

 than when they are more gracefully placed ? Why can a child walk 

 more easily with a cane than without ? Why will a book placed on 

 a desk-lid stay there while a marble would roll off ? Why is a ton 

 of stone on a wagon less likely to upset than a ton of hay similarly 

 placed? 



EXERCISES. 



Explanatory Note. The first problem in the table below may be 

 read as follows : What will be the weight of a body which weighs 

 1200 pounds at the surface of the earth, when placed 2000 miles 

 below the surface ? When placed 4000 miles above the surface ? 

 (Radius of earth =4000 miles.) All of the measurements are from 

 the surface. 





