The Wealth of Nations by Part 1 Chapter 11 Page 217

than the tithe of wheat amounted to, they were bound by capitulation to pay for the surplus at the rate of four sestertii, or eightpence sterling, the peck; and this had probably been reckoned the moderate and reasonable, that is, the ordinary or average contract price of those times; it is equal to about one-and-twenty shillings the quarter. Eight-and-twenty shillings the quarter was, before the late years of scarcity, the ordinary contract price of English wheat, which in quality is inferior to the Sicilian, and generally sells for a lower price in the European market. The value of silver, therefore, in those ancient times, must have been to its value in the present as three to four inversely; that is, three ounces of silver would then have purchased the same quantity of labour and commodities which four ounces will do at present.