42 JUNIOR GRADE SCIENCE 



(c) Examine examples of British masses, e.g. an ounce, a pound, a half 

 hundredweight. Also examine a box of metric masses, generally spoken 

 of as a box of " weights " (Fig. 33). 



FIG. 33. Box of metric weights. 



Masses are determined by means of the balance The instru- 

 ment generally used for the purpose of estimating masses is the balance 

 or pair of scales. This simple apparatus is best understood with the 

 help of the simple experiments already performed which have shown 

 that when equal masses are hung from points on a lath supported 

 at its centre, then, when the lath is at rest in a horizontal position, 

 or is in a condition of equilibrium, the distance of the masses from 

 the fulcrum or pivot will be found the same in each case. Similarly, 

 when the pans are at equal distances from the supports there is equili- 

 brium only when the masses are equal. When unequal masses are at 

 different distances from the fulcrum, there is equilibrium only when 



mass on x distance _ mass on distance 



one side from fulcrum ~~ other side x from fulcrum. 



The balance. The form of balance shown in Fig. 34 is evidently 

 similar in principle to *he supported lath with pill-boxes. This kind 

 of balance is good enough for ordinary purposes, but when exact 

 weighings are wanted, a better form, such as that already described 

 {Fig. 32) is used. All the parts in this balance are very carefully made, 

 and the greatest possible pains are taken to have very delicate supports 

 and accurate adjustments. Instead of the wooden lath described just 

 now, a brass beam is employed. This is supported at its middle line 

 on a knife edge of hard steel or agate, which, when the balance is in 

 use, rests on a true surface of similar steel. The hooks to which the 

 pans are attached are likewise provided with a V-shaped groove of 

 hard steel, which also, when the balance is in use, rests upon knife 

 edges on the upper parts of the beam. To the middle of the beam is 



