In such a work as this, however, it may sometimes be of use to compare the different real values of a particular commodity at different times and places, or the different degrees of power over the labour of other people which it may, upon different occasions, have given to those who possessed it. We must in this case compare, not so much the different quantities of silver for which it was commonly sold, as the different quantities or labour which those different quantities of silver could have purchased. But the current prices of labour, at distant times and places, can scarce ever be known with any degree of exactness. Those of corn, though they have in few places been regularly recorded, are in general better known, and have been more frequently taken notice of by historians and other writers. We must generally, therefore, content ourselves with them, not as being always exactly in the same proportion as the current prices of labour, but as being the nearest approximation which can commonly be had to that proportion. I shall hereafter have occasion to make several comparisons of this kind. In the progress of industry, commercial nations have found it convenient to coin several different metals into money; gold for larger payments, silver for purchases of moderate value, and copper, or some other coarse metal, for those of still smaller consideration, They have always, however, considered one of those metals as more peculiarly the measure of value than any of the other two; and this preference seems generally to have been given to the metal which they happen first to make use of as the instrument of commerce. Having once begun to use it as their standard, which they must have done when they had no other money, they have generally continued to do so even when the necessity was not the same. The Romans are said to have had nothing but copper money till within five years before the first Punic war (Pliny, lib. xxxiii. cap. 3), when they first began to coin silver. Copper, therefore, appears to have continued always the measure of value in that republic. At Rome all accounts appear to have been kept, and the value of all estates to have been computed, either in asses or in sestertii. The as was always the denomination of a copper coin. The word sestertius signifies two asses and a half. Though the sestertius, therefore, was originally a silver coin, its value was estimated in copper. At Rome, one who owed a great deal of money was said to have a great deal of other people’s copper.