THE BEASON TVHY 185 



1 Even a child is known by his doings, whether his work be pure, and whether 

 it be right." PEOVEEBS xx. 



of the top on to the extreme centre of the round point. When 

 the rotation subsides, and the centrifugal force is weakened, 

 then the top is no longer balanced upon the extreme point of 

 the spill, but falls upon it sides, until the force of gravita- 

 tion is exerted beyond the line of the spill, upon the body of the 

 top, and then it falls to the ground. 



816. Why does a top " sleep ?" 



Because a* that period of its spinning, which is called " sleeping," 

 me centrifugal and the gravitative forces acting upon the top, are 

 nearly balanced; and the top, obeying chiefly the rotatory force, 

 appears to be in a state of comparative rest. 



817. Wliy does the top cease to spin ? 



Because the friction of the air against its sides, and the friction 

 of the spill against the ground, act in opposition to the rotatory 

 force, which is a temporary impulse applied by external means the 

 hand of the person who spins it and as soon as this applied force 

 is expended, the top yields to the law of gravitation, which is a 

 permanent and, ever-prevailing force. 



818. Why does a marlle revolve, as it is propelled along 

 the ground? 



Because, in propelling the marble, the thumb impels the upper 

 turf ace forward, and the finger draws the under surface back- 

 card. This gives a tendency to the upper and lower hemispheres 

 of the marble to separate, which they would do, but for the 

 cohesion of the atoms of the marble. The upper part of the marble, 

 therefore, rolls forward, drawing after it the under part, which 

 acquires a forward motion by the force with which it is drawn 

 upward, and in this way the opposite portions of the marble act 

 upon each other in the successive revolutions. 



When the marble strikes upon the earth, a new influence is 

 exerted upon it, which is the friction of the earth upon the surface 

 that comes in contact with it ; but the upper part of the marble, 

 being free, overcomes the friction acting upon the Ltwer part, and 

 thus the marble continues to progress, until the applied fore* 

 ichizh projected it is expended. 



