Centre of Gravity 111 



Test the accuracy of this statement in the case 

 of the rectangle by making another hole in it and 

 suspending it again with the hook through this hole. 

 If what has been said is correct, the straight line drawn 

 in the direction of the thread should intersect the lines 

 already drawn, at the point previously determined. 



Find, in a similar fashion, the centres of gravity 

 of the other figures you have prepared. 



The true centre of gravity of each figure will not, 

 of course, be at the point of intersection of the lines 

 drawn on the surface of the millboard, but will be 

 situated at a point immediately beneath this point and 

 halfway through the thickness of the millboard. 



68. The centre of gravity of any body might be 

 found by the method just described, if it were feasible 

 to mark the exact direction of the lines of action of 

 the weight of the body when it is suspended from 

 different points on its surface. This cannot very well 

 be done in most cases, however, but we may find the 

 centre in the case of regular solids with some approxima- 

 tion to accuracy by adaptations of the above method. 



EXPERIMENT 37. Balance a boxwood metre scale 

 upon one edge of a wooden triangular prism, with the 

 flat side of the scale upon the edge of the prism. When 

 it rests in a horizontal position, its weight acting down- 

 wards must be acting in the same vertical plane as 

 the resistance of the edge of the prism acting upwards. 

 That is to say, its centre of gravity must be situated 

 at some point in the interior of the scale which is exactly 

 over the edge of the prism. Balance the scale again, 

 this time upon one of its long narrow faces. It will be 

 found that it balances about a point in its length which 

 is situated at the same distance from either end of the 

 scale as the point previously found. At some point in 



