vi PARALLEL FORCES AND CENTRE OF GRAVITY 75 



Then place a weight anywhere upon the lath and again observe 

 the reading of each balance (Fig. 27). Repeat the experiment 



FIG. 27. To illustrate Experiment GO. 



with the weight in different positions on the lath, and record 

 the results in columns as shown below : 



If columns 3 and 4 are compared after performing an experi- 

 ment of this kind they will be found to be practically the same, 

 thus again showing that when the lath is in equilibrium the sum 

 of the upward forces is equal to the sum of the downward forces. 

 The following experiments further illustrate this principle. 



EXPT. 61. Suspend a light, stiff rod by a string which 

 passes over a pulley and has attached to its other end a weight 

 equal to that of the 

 rod. 

 then 



The 



rod can 

 move as if it 

 were weightless. 

 Fasten a spring 

 balance to a con- 

 venient point on 

 the rod. Suspend 

 weights A and B at 

 the extremities of the 

 rod so that they bal- 

 ance about this 

 point, and show that 

 the magnitude of the 

 resultant is equal to the sum of the weights (Fig. 28). 



i 



a 



Via. 28. Parallel Forces. 



