are required; including the compound signs also, only fifteen. By Roman notation, without subtraction, fifteen; with subtraction, nine. By alphabetic notation, three signs without repetition. By the Arabic, one sign thrice repeated. By Federal coins, nine pieces, one of them being a repetition. By dual coins, six pieces without a repetition, a fraction remaining. In the gradation of real weights, measures, and coins, it is important to adopt those grades which are most convenient, which require the least expense of capital, time, and labor, and which are least likely to be mistaken for each other. What, then, is the most convenient gradation? The base two gives a series of seven weights that may be used: 1, 2, 4, 8, 16, 32, 64 lbs. By these any weight from one to one hundred and twenty-seven pounds may be weighed. This is, perhaps, the smallest number of weights or of coins with which those several quantities of pounds or of dollars may be weighed or paid. With the same number of weights, representing the arithmetical series from one to seven, only from one to twenty-eight pounds may be weighed; and though a more extended series may be used, this will only add to their inconvenience; moreover, from similarity of size, such weights will be readily mistaken. The base ten gives only two weights that may be used. The base three gives a series of weights, 1, 3, 9, 27, etc., which has a great promise of convenience; but as only four may be used, the fifth being too heavy to handle, and as their use requires subtraction as well as addition, they have neither the convenience nor the capability of binary weights; moreover, the necessity for subtraction renders this series peculiarly unfit for coins. The legitimate inference from the foregoing seems to be, that a perfectly practical system of weights, measures, and coins, one not practical only, but also agreeable and convenient, because requiring the smallest possible number of pieces, and these not readily mistaken for each other, and because agreeing with the natural division of things, and therefore commercially proper, and avoiding much fractional calculation, is that, and that only, the successive grades of which represent the successive powers of two.