ON BALANCES. 65 



No. 178 in the Catologue. Its peculiarity is that thin elas- 

 tic steel springs are used both for the centre of motion of the 

 balance and for suspending the pans. It is similar in prin- 

 ciple to a balance constructed by the late Dr. Steinheil of 

 Munich, in which silk ribbon was used instead of steel 

 springs. Such balances possess the advantages of simplicity 

 of construction and durability. But they are wanting in the 

 sensibility and stability requisite for a good scientific balance. 



25. I ought not to omit to mention also the beautifully 

 constructed automatic balances for testing with great accuracy 

 the weight of gold coins, invented by Mr. Napier, and now 

 used at the Bank of England and the Mints of many countries. 

 Once set in motion, these balances continue to test the weight 

 of sovereigns placed successively in one of the pans. If 

 correct, the sovereign is thrust aside into a central drawer ; 

 if heavy, into a drawer on one side ; if light, into a drawer 

 at the other side. A single man, or even a small boy only, is 

 required to keep an upright cylinder supplied with coins to 

 be tested. 



A most ingenious improvement upon these automatic 

 balances has since been made by Mr. Napier. He has added 

 a contrivance by which every heavy sovereign is shifted to 

 a second pan, where its weight is reduced to the correct 

 weight; and then it also is thrust aside into the central 

 drawer. 



26. Having thus fully described the construction of 

 balances of precision, I come to the scientific methods of 

 accurate weighing. 



The ordinary mode of commercial weighing, by putting the 

 commodity to be weighed in one scale and weights in the other 

 until an equilibirum is attained, is insufficient for scientific 

 weighings, as the results are subject to errors arising from 

 defects in the balance itself. To avoid any such errors and 

 attain scientific precision in the results, a check is required, 

 which is found in a system of double weighing, and in 

 taking the mean result of a series of successive weighings. 



27. Two methods of double weighing are commonly used. 

 One method, known as Borda's and generally used in France, 

 is that of substitution, or weighing separately two bodies to 

 be compared against a counterpoise, by placing them succes- 

 sively in the same pan, and thus ascertaining their difference 

 in divisions of the index. The second method, known as 



VOL. II. F 



