MASS AND WEIGHT 



27 



In the metric system a name is given to the mass of water which 

 will exactly fill a cubic centimetre at a temperature of 4 C. It is 

 called a gram. The same prefixes are used to express fractions and 

 multiples of a gram as have been employed in the case of the metre 

 and litre. The kilogram, or unit of mass, is one thousand times greater 

 than that of a gram, and is the unit in use for ordinary purposes. 



METRIC MEASUREMENT OF MASSES. 



10 milligrams = 1 centigram. 10 grams = 1 dekagram. 



10 centigrams = 1 decigram. 10 dekagrams = 1 hektogram. 



10 decigrams = 1 gram. 10 hektograms = 1 kilogram. 



Mass is not weight. If a mass of 1 Ib. is 

 dropped from the hand it falls to the ground. 

 If the same mass is hung upon the end of a 

 coil of iron wire, the coil is made longer by the 

 downward pull of the mass fixed to its end. 

 The amount by which a steel spring is length- 

 ened, as the result of such downward pull of 

 masses attached to its end, is used to measure 

 their weights in the instrument called a spring 

 balance. If a delicate balance of this kind like 

 those used in weighing letters, is used, the 

 weight of a small piece of iron hung on to the 

 balance can be made to appear greater by 

 holding a strong magnet beneath it. But, 

 though the weight may appear greater, the 

 mass or quantity of matter is, of course, the 

 same whether the magnet is under the iron or 

 not (Fig. 21). 



There is thus a clear distinction between 

 mass and weight ; for mass signifies quantity 

 of substance, while weight is a force. In other 

 words, the mass of a thing is the quantity of 

 matter in it, and its weight is the force with 

 which the earth pulls it. Unsupported things 

 fall to the ground ; a fact which can also be 

 expressed by saying that they are attracted 

 to the earth. Now, even when they are sup- 

 ported. like the objects on a table, the earth 

 attracts them just as much, only the table 

 prevents them from falling, as they would do 

 if there were no table there. The force with 



FlG . 21 ._ The ^ weight 

 may be made to appear 



