34 MECHANICS. 



more than two or three feet high, will run over the top. 

 This is owing to the capillary attraction in the minute 

 pores of the bladder, drawing the water within it faster 

 than the same attraction draws the alcohol outward. One 

 liquid will thus intrude itself into another with great 

 force. A bladder filled with alcohol, with its neck tightly- 

 tied, will soon burst if plunged under water. If a blad- 

 der is filled with gum-water, and then immersed as before, 

 the water will find its way within against a very heavy 

 pressure. 



In this manner sap ascends through the minute tubes in 

 the body of trees. The sap is thickened like gum-water 

 when it reaches the leaves, and a fresh supply, therefore, 

 enters through the pores in the spongelets of the roots by 

 capillary attraction, and, rising through the stem, keeps 

 up a constant supply for the wants of the growing tree. 



CENTRE OF GRAVITY. 



The centre of gravity is that point in every hard sub- 

 stance or body, on every side of which the different parts 

 exactly balance each other. If the body be a globe or 

 Fig. la round ball, the centre of 



gravity will be exactly at the 

 g' l,.'-'-'^ centre of the globe ; if it be 

 a rod of equal size, it will be 

 at the middle of the rod. If 

 a stone or any other sub- 

 stance rest on a point directly under the centre of gravity, 

 it will remain balanced on this point ; but if the point be 

 not under the centre of gravity, the stone will fall toward 

 the heaviest side. 



Some curious experiments are performed by an ingenious 

 management of the centre of gravity. A light cylinder 

 of cork or pasteboard contains a concealed piece of lead, 

 g (fig. 18). The lead, being heavier than the rest, will 



i 



