THE ELEMENTS OF MACHINERY. 



97 



TVTiat are the 

 disadvantages 

 of a compound 

 lever ? 



Describe the 

 common steel- 

 yard. 



208. The disadvantasre of a compound lever 

 is, that its exercise is limited to a very small 

 sj)ace. 



209. The different varieties of weighing machines are varie- 

 ties or combinations of levers. The common steel-yard is a 

 lever of unequal arms, belonging to the first class. It consists 

 of a bar (Fig. 67) marked with notches to indicate pounds and ounces, and a 

 weight wliich is movable along the notches. The bar is furnished with threa 

 hooks, or ringsj on the lai'gest of which the article to be weighed is always 

 liung. The other hooks serve to support the instrument when it is in use, 

 and the pivot by which they are attached to the bar serves as the fulcrum. 

 The weight, Q, shding upon tlie bar, balances the article, P, which is to be 

 weighed, it being evident that a pound weight at D wiU balance as many 

 pounds at P as the distance A C is contained in the space D C. 



Tig. 67, 



It may happen that when the weight Q is moved to the last notch upon the 

 bar B C, that the article P will still preponderate. In tliis case, the steel-yard 

 is held by the hook or ring nearer to A, which hangs down in the figure, and 

 the steel-yard turned over, it being furnished with two sets of notches on 

 opposite sides of the bar. Ey this means the distance of P, the article weighed, 

 from the fulcrum is diminished, and the weight Q, at the given distance upon 

 the opposite side of the fulcrum, wOl balance a proportionally greater resist- 

 ance, or weight. 



Describe the ^^^' '^^^ Ordinary balance is a lever of the first class, with 



ordinary bal- equal arms, in which the power and the weight are neces- 

 *"''*■ sarily equal Fig. G8 shows the common form. The fulcrum 



or axis, is made wedge -like, with a sharp kuife-like edge, and rests upon a 



