OF THE MECHANICAL POWERS. 57 



.again ; and therefore there is an equilibrium between LECT. 

 thejn. So, if a power equal to 2 ounces be applied at ^^ -^. 

 the distance of 6 inches from the prop, it will just 

 balance the weight W; for 6 multiplied by 2 is 12, as 

 before. And a power equal to 3 ounces placed at 4 

 inches distance from the prop would be the same ; for 

 3 times 4 is 12 ; and so on, in proportion. 



The statera or Roman steelyard is a lever of this kind, The steel- 

 and is used for finding the weights of different bodies 

 by one single weight placed at different distances from 

 the prop or center of motion D For, if a scale hangs 

 at A y the extremity of the shorter arm AB, and is of 

 such a weight as will exactly counterpoise the longer 

 arm B C ; if this arm be divided into as many equal 

 parts as it will contain, each equal to AB, the single 

 weight P (which we may suppose to be 1 pound) will 

 serve for weighing any thing as heavy as itself, or as 

 many times heavier as there are divisions in the arm 

 BC, or any quantity between its own weight and that 

 quantity. As for example, if P be 1 pound, and placed 

 at the first division 1 in the arm BC, it will balance 1 

 pound in the scale at A : if it be removed to the second 

 division at 2, it will balance 2 pounds in the scale : if 

 to the third, 3 pounds ; and so on to the end of the 

 arm BC. If each of these integral divisions be subdi- 

 vided into as many equal parts as a pound contains 

 ounces, and the weight P be placed at any of these 

 subdivisions, so as to counterpoise what is in the 

 scale, the pounds and odd ounces therein are by that 

 means ascertained. 



To this kind of lever may be reduced several sorts of 

 instruments, such as scissars, pincers, snuffers ; which 

 are made of two levers acting contrary to one another : 

 their prop or center of motion being the pin which 

 keeps them together. 



In common practice, the longer arm of this lever 

 greatly exceeds the weight of the shorter ; which gains 

 ?reat advantage, because it adds so much to the power . 



