20 PRACTICAL LESSONS IN SCIENCE. 



or cube of uniform density the center of gravity is at the cen- 

 ter of the body, while in a pyramid or cone it is below the 

 center of the body. The lower the center of gravity the more 

 stable the body. A load of hay may be overturned more easily 

 than a load of stone, as the center of gravity is lower in the load 

 of stone. A body will stand up as long as a vertical line from its 

 center of gravity falls within the base of the body. It is easier 

 to stand on our feet than on a pair of stilts, because, in the 

 latter case, the slightest motion will throw the vertical line out- 

 side the supporting base. 



It is difficult to illustrate the second law of gravity, but the 

 intensity of light and heat vary according to the same law, and 

 it can be more easily illustrated when treating of those subjects. 



The weight of a body is the downward pressure which it can 

 exert, and this pressure or weight is due to the action of gravi- 

 tation exerted by the earth. Weight depends upon gravity, 

 measures gravity, and varies according to the laws of gravity. 

 On the surface of the earth the variations of gravity are so 

 slight that weight is usually considered as a good measure of the 

 mass or quantity of matter in a body as well as of the force of 

 gravitation. As the earth is eomewhat flattened at the poles, 

 the distance to the center of gravity of the earth is less at the 

 poles than at the equator, consequently the force of gravity is 

 -greater and a body will weigh more at or near the poles than at 

 or near the equator. Water, dirt or mud is thrown from a rap- 

 idly turning wheel; the earth as a rotating body tends to throw 

 off matter from the equatorial regions; this tendency also less- 

 ens the force of gravity and consequently the weight of bodies 

 at the equator. The combined effect of these two causes is to 

 make gravitation at the equator less by about 1-192 part of 

 its value at the poles. Conversely, the increase of the force of 

 gravity from the equator north and south, as shown by the 

 vibrations of the pendulum, indicates a flattening of the earth 

 toward the poles. 



The quantity of space a body of matter occupies is its volume, 

 and is expressed in cubic inches or feet. The quantity of matter 



