30 SCIENCE PRIMERS. [MATERIAL 



weight into one, that one goes down and the other 

 rises. If now you either pull or push the empty 

 scale downwards, the beam may be brought into the 

 horizontal position again, and the effort required to 

 bring it into the horizontal position will be the 

 greater, the greater the weight of the body in the 

 opposite scale. An ounce in the one scale is easily 

 raised by the pressure of a finger in the other. A 

 pound requires more effort; ten pounds needs put- 

 ting out the strength of the arm ; to raise fifty pounds 

 involves still more exertion ; while a couple of hundred- 

 weight will not be stirred by the strongest push or pull 

 upon the empty scale. 



Suppose that, instead of pressing down the empty 

 scale, you put something that has weight into it; 

 then, as soon as this weight is equal to that in the 

 other scale, the beam will become horizontal. In fact, 

 one scale has just as much tendency to move towards 

 the centre of the earth as the other has, and as neither 

 can go down without pulling the other up, they neu- 

 tralise one another. It comes to the same thing, as if 

 two boys of equal strength were pulling one against 

 the other ; so long as the pulls in opposite directions 

 are equal, of course neither boy can stir ; while the 

 smallest addition of strength to one enables him to 

 pull the other over. 



22. The Weight of the same Bulk or 

 Volume of Water is Constant under the same 

 conditions. Mass. Density. 



Now let two graduated thin glass measures be put 

 into the two scales, and made to counterpoise one 

 another exactly. Then, if even a single drop of 



