52 ELEMENTARY GENERAL SCIENCE CHAP. 



box, and try it in the pan with the other. If the two together 

 are too heavy, take out the smaller weight and put in the one 

 below it, and so on, adding one weight after the other, without 

 missing any, until you find the correct weight. When you 

 have completed the weighing, write down and add up the 

 weights that are missing from their places, and check the 

 figures as you put each weight back in its place. 



EXPT. 55. To become familiar with the process of weighing, 

 find the mass of half-a-crown, a shilling, and other suitable 

 bodies. 



EXPT. 56. Find the mass in grams, c., of an ounce weight. 



The Weight of a Body is not determined by an ordinary 

 Balance. The spring balance, and not a pair of scales, must be 

 used to determine the weight of a mass at any place, because by 

 'means of the latter apparatus all we do is to make a comparison 

 between the weight of an unknown and that of a known mass. 

 Though the weight of the known mass (say the 50 gram weight 

 out of the box) varies from place to place, its mass remains con- 

 stant, and what we determine when we effect a weighing with 

 the balance is the mciss of the body experimented upon. If we 

 could attach the 50 gram weight to a very sensitive spring 

 balance and carry it from the equator to the pole, we should 

 find, as we have already seen, that its weight as recorded by the 

 spring balance would vary continuously. Hence we estimate the 

 equality of masses by the equality of their weights ; the weight 

 of the body whose mass is to be determined must, when the 

 balance is in equilibrium, be equal to that of the known mass, 

 and consequently we can argue that their masses are equal. By 

 the balance we measure the mass of a body, whereas a spring 

 balance enables us to determine its weight at a given place. 



When a Balance is in equilibrium the moments, about the 

 point of suspension, of the forces equal to the weight of the masses 

 are equal. When the balance is true the arms are of equal 

 lengths, and consequently the weights are equal also. 



CHIEF POINTS OF CHAPTER, IV. 



The Mass of any body is the quantity of matter it contains. 



The Imperial Standard Pound Avoirdupois is the amount of matter 

 contained in a lump of platinum of a certain size which is kept at 

 the Exchequer Chambers. 



The metric standard of mass is the Gram ; but in everyday use 

 the kilogram, which is equal to a thousand grams, is employed. 



