GRAVITATION. 55 



say, the centre of gravity must be raised a distance equal 

 to the difference between ga and gc, or the distance nc. 

 But to lift g this distance is the same as to lift the whole 

 brick vertically a distance equal to ne. Now draw similar 

 figures for the brick when placed upon its edge and upon 

 its end. In each case make gn equal to ga, and see that 

 the value of nc decreases. But nc represents the distance 

 that the brick, or its centre of gravity, must be raised 

 before the line of direction can fall without the base, and 

 the body be overturned. To lift the brick, or its centre of 

 gravity, a small distance involves less work than to lift it 

 a greater distance. Therefore, the greater the value of nc, 

 the more work required to overturn the body, or the 

 greater its stability. But this greater value of nc evidently 

 depends upon a larger base, a lower position for the centre 

 of gravity, or both. 



FIG. 25. 



(a.) These facts explain the stability of leaning towers like those 

 of Pisa and Bologna. In some such towers the centre of gravity 

 is lowered by using heavy materials for the lower part and light 

 materials for the upper part of the structure. It is difficult to stand 

 upon one foot or to walk upon a tight rope because of the smallness 



