But if the term has been generally understood in this way, people must have been fully aware that value was essentially different from length: they would know perfectly well that a piece of cloth of a yard long would continue to be a yard long when it was sent to China; but that its value, that is, its general power of purchasing in China, or the estimation in which it was held there, would probably be essentially altered. But allowing this most marked distinction, and that the value of a commodity cannot be so well defined, and its variations so accurately measured, as the length of a commodity—where is the false analogy of endeavouring to measure these variations as well as we can? We cannot certainly describe the wealth of a merchant, nor measure the increase of his wealth during the last four years, with the same exactness as we can describe the height of a boy, and measure the amount of his growth during the same period. We can perform the latter operation with the most perfect precision by means of a foot-rule. The nature of wealth, and the best instruments used to measure its increase, are such, that the same precision is unattainable; but there is no false analogy involved in the process of measuring the wealth of a merchant at one time with his wealth four years before, by the number of pounds sterling which he possesses now, as compared with the number of pounds sterling he possessed at the former period. What false analogy is involved in applying money to measure the value of the advances of a manufacturer, as compared with the value of his returns, in order to estimate his profits? and what can the author mean by saying, that no relation of value can exist between commodities at different periods;[71] and that it is a case where money has no function to perform? Notwithstanding such assertions, we see every day the most perfect conviction prevailing among all agriculturists, merchants, manufacturers, and shopkeepers, and among all writers on political economy, except the author, that to estimate the relation of commodities, at different periods, in regard to their general power of purchasing, and particularly the power of purchasing labour, the main instrument of production, is a most important function, which it is peculiarly desirable to have performed; and that, for moderately short periods, money _does_ perform this function with very tolerable accuracy. And for this specific reason; that, for moderately short periods, a given quantity of money will represent, more nearly than any other commodity, the general power of purchasing, and particularly the power of setting labour in motion, so vital to the capitalist. It will approach, in short, more nearly than any other commodity, to that invariability which the author thinks so utterly useless in a measure of value, and the very mention of which seems to excite his indignation.[72]