48 Domestic Science 



of the tea and the lead are also equal. The volumes 

 occupied by the two bodies are very different but the 

 " quantity of matter " in each is the same. We are 

 thus led to the idea that, if we have a method of com- 

 paring the weights of any two bodies, we are at the 

 same time able to compare their respective masses. 

 This point is of importance since, when we purchase 

 tea, sugar, butter and other commodities by weight, as 

 we usually term it, we are not primarily concerned 

 with the pull exerted between the Earth and the article 

 purchased, but with the quantity of the article which we 

 obtain. The instrumentin general use for the comparison 

 of weights and consequently of masses is the balance. 

 Many forms of this exist but the principle upon which 

 the working of each depends is the same in all cases. 



Fig. 19. 



EXPERIMENT 13. Bore a hole through a long 

 wooden rod a boxwood metre scale will do excellently 

 carefully at right angles to its length and midway 

 between the two ends. Its diameter should be such 

 that a knitting-needle, when pushed through, fits closely. 

 Support the ends of the needle upon two thick wooden 

 blocks or large books and note whether the rod 

 remains in a horizontal position. If it does not, fold 

 a piece of lead foil the sort in which chocolates are 

 wrapped will serve around the end which is higher, 

 and, by sliding the foil along the rod, find a position 



