MASS AND ITS MEASUREMENT 



no doubt about the reality of the attraction which bodies exert 

 upon one another. So long ago as 1798 Cavendish measured the 

 force of attraction between two lead balls, and his experiments 

 have been repeated by a number of other investigators. 



The weight of a body is the force with which it tends to 

 move towards the earth. We see at once the difference there 

 is between mass and weight. One is matter, the other is a 

 force. When, we speak of a mass of one pound we have seen 

 that it means nothing more than the amount of matter in a 

 certain lump of iron or other material ; but when we speak of the 

 weight of one pound we mean the force with which 

 the mass of a pound tends to move towards the earth. /^\ 

 The student will do well to clearly understand this y-y 

 distinction before proceeding. 



EXPT. 46. Procure a spring balance (Fig. 20) 

 and attach it to a rigid support. Notice that to 

 bring the index down you must exert a pull on the 

 bottom hook. The greater the pull the farther 

 down the scale does the index move. 



EXPT. 47. Attach any convenient mass to the 

 hook of the balance, and notice that the index 

 moves a certain distance down the scale as in the 

 last experiment. The pull in this case measures 

 the earth's attraction for, or the weight of, the 

 mass at the place of observation. 



EXPT. 48. Repeat the last experiment, using 

 different masses, and notice that the weight of the FlG ; 2 9-~ A 

 masses, as measured by the pull on the spring, Balance, 

 varies in the different cases. Whenever the pull 

 on the spring is the same, we know the masses are equal, and 

 consequently the equality of masses can be tested by the 

 equality of their weights. 



The Weight of a given Mass varies from place to place. 

 Bearing the definition of weight in mind, it will be clear from 

 Newton's law of gravitation that since a mass is farther away 

 from the earth (which acts exactly as if its whole mass were 

 collected at its centre) when it is on the top of a mountain than 

 when at the sea-level, the weight of this mass ought to be more 

 at the sea-level, for it is there nearer the centre than at the 

 mountain top. This is found to be the case, but to actually 



