112 Domestic Science 



the vertical plane, containing the edge of the prism, 

 above this position of rest is its centre of gravity. If 

 it were possible to balance the scale upon one end, 

 a third vertical plane could be found containing the 

 centre of gravity. The three planes thus determined 

 have only one common point of intersection, and this 

 must be the centre of gravity of the scale. By making 

 the assumption, however, that the middle part of the 

 scale is composed of homogeneous matter, we may con- 

 sider the centre to be midway along the portion of 



(*) 



w 



(ft) 



Fig. 33. 



the line which forms the intersection of the two planes 

 found by actual experiment, contained within the scale. 

 69. It will probably have been noted that it is 

 a much more difficult operation to balance the scale 

 upon one of its long narrow faces than upon one of 

 the flat ones, and that the feat of balancing it upon one 

 of its end faces seems impossible. We express the ideas 

 derived from these observations by saying that, when 

 supported upon a flat face, the scale is in a more stable 

 position than when balanced about a narrow edge, and 



