106 CENTRE OF GRAVITY. 



bodies gravitate towards each other inversely as 

 the squares of their distances, that is, if one body 

 is at the distance two, and another at the distance 

 three, the body at two will, on account of distance, 

 gravitate as nine, while the body at three, gravitates 

 as four. We must not, however, misunderstand 

 what is meant by distance from a body in respect 

 of gravitation. It is not from the surfaces of the 

 bodies that the distance is estimated, it is from 

 their centres of gravity, or of weight. Those 

 centres, in bodies, are the points into which the 

 whole would be squeezed, if the different parts of 

 the body itself gravitated so strongly as to be able 

 to reduce the whole to one point ; and they may or 

 may not be the measured centres of bulk in the 

 bodies. In a perfect globe or round ball of uni- 

 form matter, the centre of gravity is the same 

 as the centre of the ball; but if half the ball 

 were lead, and the other half cork, the centre 

 of gravity would be so far into the leaden half, 

 that it would scarcely be possible to roll the ball, 

 and it would always lie on the centre of the 

 leaden surface. Very amusing toys for children are 

 made upon that principle, by carving little figures 

 in the pith of elder, or any other very light sub- 

 stance, and glueing a half leaden bullet on the 

 bottom. The figures are so much lighter than 

 the lead, that they get up again when they are 

 upset. Toys for children of an elder growth, 

 such as bowls for playing on the green, are loaded 

 in the same manner, which gives them a bias, 

 or makes them run crooked; so that an expert 

 player can bring his bowl in at a side, and take 

 the mark away from a bowl that touches it. On the 

 same principle, ships' boats are ballasted, to pre- 



