Page:Popular Science Monthly Volume 37.djvu/360

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every kind of business in some way contributes its share. There is no difficulty whatever in getting these things, for the merchants 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 carefully 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 sensitive to five milligrammes, either loaded or empty. It is represented in Fig. 1. The beam, support, pointer, and index were cut out of wood. The scale-pans were tin-box tops. The knife-edges

PSM V37 D360 Balance of equal arms.jpg
Fig. 1.—Balance of Equal 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-