34 



WELLS'S NATURAL PHILOSOPHY. 



"VNHiat is a Sys- 

 tem ofWeiiihts 

 and Measures? 



lead is drawn to the earth with a force exactly equal to the superiority of its 



mass over that of the feather. If, 

 however, we tie on a suflficient 

 number of feathers to make up a 

 quantity of matter equal to that 

 of the lead, the equilibrium is re- 

 stored — the two quantities are 

 attracted with equal force, and 

 the beam is supported in a hori- 

 zontal position. 



65. In all the opera- 



\ tions of trade and com- 



\ merce, we sell, or ex- 



Oi change a given quantity 

 \ / of one article or substance 



"> — '' fQj. a^ certain quantity of 

 Bome other article or substance — so much flour for so much 

 sugar, or so much sugar and flour for so much gold. 

 Hence the necessity, which has existed from 

 the earliest ages, of having some fixed rules or 

 standards, according to which different quanti- 

 ties of difierent substances may be compared. A set, or 

 series, of such rules or standards of comparison, is called a 

 System of Weights and Measures. 



Various nations adopt different standards, but in the civil- 

 ized and commercial world, but two great Systems of Weights 

 and Measures are generally recognized. These are known as 

 the English, and the French Systems. 



In the English System, which Ls the one used in the United 

 States, there are two systems of weights — Troy and Avoirdu- 

 pois "Weight Troy Weight is principally used for weighing 

 gold and silver ; Avoirdupois for weighing merchandize, other 

 than the precious metals. It derives its name from the French avoirs (averia\ 

 goods or chattels, and pouls, weight. The smallest weight made use of ia 

 the English Sy.stem is a grain. By a law of England enacted in 1286.lt 

 was ordered that 32 grains of wheat, well dried, should weigh a pennyweight 

 Hence the name grain applied to this measure of weight. It was afterward 

 ordered that a pennyweight should be divided into only 24 grains. Grain 

 weights for practical purposes, are made by weighing a thin plate of metal of 

 uniform thickness, and cutting out, by measurement, such a proportion of 

 the whole as should give one grain. In this way, weights may be obtained 

 for chemical purposes, which weigh only the 1,000th part of a grain. 



What are the 

 two great SyB- 

 tems of 

 Weights and 

 Measures ? 



What are the 

 ppculiarities of 

 the English 

 System f 



