46 ELEMENTS OF SCIENCE 



quite stable namely, that of an ingeniously contrived 

 oscillating pendulum. 



A body is in equilibrium, or a state of rest, when the 

 forces which act upon it counterbalance one another. 

 One such force may be the force of resistance which a 

 supporting body offers to the weight of the body which 

 it supports and so keeps in equilibrium. 



As numbers and quantities may be represented by 

 arithmetical or algebraic symbols, so forces may be 

 represented by lines of definite lengths. These will be the 

 longer the greater the force they represent, and they will 

 also serve to indicate the direction of the forces. 



If two forces be in equilibrium, they must be equal in 

 magnitude and opposite in direction. It is plain that 

 if such were not the case, the greater of the forces would 

 overcome the other, therefore the two would not neutralise 

 each other, and so we should have motion, and not 

 equilibrium. 



But whatever the number and direction of the forces 

 which may act upon any point, they can only produce 

 motion in one direction. This is called the resultant of 

 such forces, which are the several components of this 

 resultant. 



When two or more forces act on a point in the same 

 direction, the resultant must be equal to their sum, and 

 if in opposite directions, then to the difference between 

 their sums. 



Thus if any point be pressed upwards by a force of 

 ten pounds and downwards by a force of five pounds, the 

 resultant must be a pressure upwards of five pounds. If 

 the pressure towards the west be 3 + 5 + 9, while that 

 to the east be 7 + 6 + 4 the resultant =17-17 or o, 

 which is equilibrium. 



As to direction, let the equal forces A and B act 



