50 MECHANICS. 



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. 



SECTION III. 



CENTRE OF GRAVITY. 



The centre of gravity is that point in every hard 

 substance or body, on every side of which the different 

 parts exactly balance each other. If the body be a 

 globe or round ball, the centre of gravity will be ex- 

 actly at the 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 substance 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 wiU fall toward the heaviest side. 



Some curious experiments are performed by an in- 

 genious management of the centre of gravity. A 

 light cylinder of cork or 

 pasteboard contains a con- 



9^ 2 -^ cealed piece of lead, g 



{Figure 18). The lead, 

 being heavier than the rest, 

 will cause the cylinder to 

 roll up an inclined plane, when placed as shown by the 



