CENTRE OF GRAVITY. 



33 



mass ; viz., at the bases, about one-fourth of the distance between the apex 

 and the centre of gravity of the base. 



When the centre of gravity of any body is supported, that body cannot 

 fall. So the well-known leaning towers are perfectly safe, because their lines 

 of direction fall within the bases. The centre of gravity is in the centre of 

 the leaning figure. The line of direction drawn vertically from that point 

 falls within the base ; but if the tower were built up higher, so that the centre 

 of gravity were higher, then the structure would fall, because the line of direc- 

 :ion would fall without the base. 



We see that animals (and men) are continually altering the position of 



Fig. 27. Balancing a weight on a nail and key. 



the centre of gravity ; for if a man bears a load he will lean forward, and if 

 he takes up a can of water in one hand he will extend the other to preserve 

 his balance or equilibrium. 



The experiment shown in the accompanying illustration is apparently 

 very difficult, but it will be found easy enough in practice if the hand be 

 steady. Take a key, and by means of a crooked nail, or " holdfast," attach 

 it to a bar of wood by a string tied tightly round the bar, as in the picture. 

 To the other extremity of the bar attach a weight, and then drive a large- 

 headed nail into the table. It. will be found that the key will balance, and 

 even move upon the head of the nail, without falling. The weight is under 

 the table, and the centre of gravity is exactly beneath the point of suspension. 



