UNITS OF WEIGHT 



11 



FIG. 10. 

 Trip Scale. 



use measuring rods, tapes, chains, etc., and to get 

 accurate weights we use balances and scales. 



If the balance has two arms of equal length, as in 

 the chemical balance (Fig. 9) and 

 the "trip" balance of the labora- 

 tory (Fig. 10), the weights used as 

 the counterpoise must be exactly as 

 heavy as the object weighed. The 



object to be weighed on 



such balances is placed in 



the left-hand pan, and the weights in the right. 



In making the spring balance (Fig. 11) the manufac- 

 turers stretch the spring by means of weights placed on 

 the hook, and then mark the successive positions of the 

 pointer on the graduated scale. A body which stretches 

 FIG. 11. the spring to the same extent must be just as heavy as 

 the weights used. Scales for weighing heavy objects, 

 such as cars, or loads of hay or coal, are made by putting together a 

 system of levers (cf. 198), so that the whole apparatus can be kept 

 in a small space under the weighing platform. 



13. Units of Weight. Man has used many units of 

 weight. The old English pound (from the Latin pondus, 

 a weight) was originally the weight of 7680 grains of wheat 

 "all taken from the middle of the ear, and well dried." 

 From this origin of the pound came the word "grain," 

 the small division of the pound. Henry VIII, King of 

 England from 1509 to 1547, made the "avoirdupois" 

 pound the unit of weight. It contains 16 ounces, or 7000 

 grains. In 1758 a piece of brass of suitable size was de- 

 clared by Parliament to be a pound. Later, a piece of 



