62 MECHANICS. 



just five inches from your fulcrum, and it will balance 

 your five-pound weight ; or, if your poise weigh half a 

 pound, you must place it just double that distance from 

 your fulcrum, or hook, held in your hand. If your 

 steelyards will not stand the test of your measure, they 

 are false. But in order to make the steelyard conform 

 to this rule, the short arm should exactly balance the 

 long one. This is not often the case, and, instead of 

 putting on more weight at the short end, it has been 

 usual to reduce the poise, or weight, on the long arm. 

 This would amount to exactly the same thing, if the 

 poise continued stationary, and equally distant as the 

 weight from the pivot ; for it is obvious, as in all 

 equations, that adding to one scale is the same thing as 

 deducting from the other ; but, to be equal, both scales 

 must be equally distant from the pivot on which they 

 turn. Now the poise in the steelyard is one of the 

 scales, and this is continually shifting ground;, when, 

 therefore, this poise is five times as far from the pivot 

 as the weight is, one ounce added to it, or taken from 

 it, makes five times the diiference that it would do 

 when five times nearer. Thus, by reducing the poise, 

 the simplicity of the steelyard is totally deranged, for 

 you must graduate your indents and figures by a com- 

 plicate rule, or your balance is obviously wrong. 



To avoid this difficulty, the balance weight is fixed 

 on the side with the main weight, or, what is the same 

 thing, a scale, heavy enough to balance the poise end 

 is used, and the steelyard has now become a balance, 

 and will give you the weight as accurately as two 

 scales with fifty-six pound weights in one of them. And 

 it is now extremely easy to detect a false balance : you 

 have only to weigh the poise and measure the distance 

 of the indents and figures on the long arm, or scale 

 beam. It is on this simple principle, this plain rule of 

 simple proportion, that our largest scales for weighing 

 hay, &c. are or should be constructed: 1 pound, 50 

 inches from the pivot, balancing 50 pounds hanging one 



