432 Fly-rods and Fly-tackle. 



It is a good idea to test one's spring-balance when 

 or soon after it is bought, and periodically afterwards. 

 I test mine at least once every season. It is best done 

 by comparison with a balance scale not with another 

 spring-balance, the accuracy of which may be unknown. 

 A good grocer's scale reading to ounces will answer, but 

 not a druggist's scale, since the apothecary's ounce is 

 heavier and his pound lighter than the avoirdupois 

 ounce and pound, to which the spring-balance is grad- 

 uated. I proceed as follows : I place a small tin pail 

 or similar receptacle on the grocer's balance, and see what 

 it weighs. Let us say it is short of half a pound. I 

 then set the balance to half a pound, and slowly run 

 water into the pail until it balances exactly. I then 

 weigh the pail and its contents on my spring-balance, and 

 see whether it indicates the same weight. If it does, 

 then I return the pail to the grocer's balance, set it to a 

 pound, and again add water until it balances, and try my 

 spring-balance again; and so on throughout its range. 



The celebrated scientist, Sir Humphry Davy, men- 

 tions a method of determining the weight of trout from 

 their length, in his Salmonia ; or, Days of Fly-Fishing^ 

 published in 1828. It proceeds on the mathematical 

 principle that solids of the same shape are to each 

 other as the cube of their dimensions. In other words, 

 if we know just how long a pound trout is, we can close- 

 ly calculate the weight of a trout of any other length. 

 The problem is worked out by cubing the length in 

 inches of the unknown trout, and dividing this result 

 by the cube of the length of the pound trout. This 

 gives the weight of the unknown trout in pounds or 

 fractions of a pound. 



