116 GRAVITATION OF DISTANCE. 



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 uniform 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 substance, and gluing 

 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 prevent them 

 from upsetting by the action of the wind on their 

 sails ; and coaches that have a box below, for heavy 

 luggage, are much safer than those that carry a 

 much smaller weight of luggage on the top. 



The gravitation of distance, or of the position 

 which one body has with regard to another, depends, 

 like specific gravity, on two elements, the absolute 

 weight of the body, and the distance ; and it varies 

 with every change in either of these. It is inversely 

 as the square of the distance, and directly as the 

 absolute gravity or quantity of matter. But the 



