16 SCIENCE OF COMMON THINGS. 



Centre of gravity in man and animals. 



If he carried the load in the position of A, Fig. 1, he would fall back- 

 wards, as the direction of the centre of gravity would fall beyond his 

 heels ; to bring the centre of gravity over his feet, he assumes the posi- 

 tion indicated by B, Fig. 2. 



91 When a person carries a load upon his head, why is it necessary 

 to stand perfectly upright ? 



In order tliat the centre of gravity may be over his 

 feet. i 



Q& Why does a person in rising from a chair bend forward ? 



When a person is sitting, the centre of gravity is 

 supported by the seat ; in an erect position, the centre 

 of gravity is supported by the feet / therefore, before 

 rising it is necessary to change the centre of gravity, 

 and by bending forward we transfer it from the chair 

 to a point over the feet. 



93 Why does a quadruped, in walking } never raise loth feet on the same 

 side simultaneously f 



Because, if it did, the centre of gravity would be un- 

 supported, and the animal would tend to fall over. 



94 Why is a large turtle placed on its back unable to move f 



Because the centre of gravity of the turtle is, in this 

 position, at the lowest point, and the animal is unable 

 to change it ; therefore it is obliged to remain at rest. 



95 WJiy is it more difficult to overthrow a body Jiaving a broad base 

 than one resting upon a narrow basis f 



Because a body cannot fall over, so long as a line 

 directed from the centre of gravity vertically towards 

 the surface upon which the body rests, falls within the 

 figure formed by the base of the body in question. 



