34 6 



THE POPULAR SCIENCE MONTHLY, 



every kind of business in some way contributes its share. There 

 is no difficulty whatever in getting these things, for the mer- 

 chants and mechanics are usually pleased when the boys ask 



for them. 



One of the pieces of apparatus made in class was a steelyard, 

 which was constructed of a foot-rule. Exactly one inch from the 

 end a hole was bored ; through this a wire was passed and bent 

 into a loop. This served for the pivot. Three fourths of an 

 inch from the same end another wire, similarly bent and inserted, 

 served for the suspension of the weight. The pea was made of a 

 small piece of pig-iron picked up at the furnace. It was care- 

 fully weighed, and had a small cord tied around it so that it could 

 be slid along the beam of the steelyard. This apparatus was 

 made to illustrate the lever of the first order, and when tested 

 weighed as accurately as the grocer's scales. 



Another piece was a balance of equal arms, which was sensi- 

 tive to five milligrammes, either loaded or empty. It is repre- 

 sented in Fig. 1. The beam, support, pointer, and index were cut 

 out of wood. The scale-pans were tin-box tops. The knife-edges 



Fig. 1.— Balance op Eqttal Arms. 



were made of old umbrella wire tempered hard. The weights, 

 from ten grammes to five milligrammes, were made of pieces of 

 copper wire. In making this balance, the pupil had his attention 

 forcibly called, by repeated failures, to the necessity of having 

 the arms exactly equal, to the best position of the center of 

 gravity of the balance, and to the importance of the knife-edges. 

 This balance — the best of four brought in — was used to determine 

 the specific gravities of minerals, and the results obtained agreed 

 closely with those given in Dana's Manual. It was also used in 

 the candle experiment to show that there is gain in weight when 

 a candle burns, and for numerous other experiments. 



A hydrometer, made by another boy in the same class, accord- 



