40 GENERAL SCIENCE 



corner by a pin stuck through a hole near its edge. Hang a plumb 

 line from the pin and draw a line ba (Figure 31) parallel to and 

 directly under the plumb line. Now hang the piece of metal from 

 another point c and draw another line cd in the same way that the 

 line ab was drawn. The intersection of these jtwo lines will be the 

 center of gravity of the piece of metal. It should balance at this 

 point, and if a needle is passed through a hole made at this point, 

 the metal should remain at rest in whatever position it is placed. 



The center of gravity of the earth is somewhere near 

 the geometrical center of it. The pull of the earth for 

 mathematical purposes may be considered to be at that 

 point. Now an object at the poles is about thirteen 

 miles nearer to the center of the earth than an object 

 at the equator, and as a consequence an object will weigh 

 more at the poles, the difference being about one part 

 in 590. What will be the effect on the weight of objects 

 of taking them up in the air several miles ? 



Objects are said to be in " stable " or " unstable " 

 equilibrium according to the position of the center of 



FIG. 32. Stable and Unstable Equilibrium. 



gravity of the mass (Figure 32). An object is in its most 

 stable position when its center of gravity is as low as 

 possible. In what position will a sphere be in most stable 

 equilibrium? A cylinder? A cone? 



Effect of Air on Falling Bodies. A stone and a feather, 

 if dropped from the top of a tall building, will not reach 

 the earth in the same time. The reason is that the air 

 resists being pushed out of the way, and the heavier 

 object more easily overcomes this resistance. If the air 



