the number of days' food. The day's food is the common measure of value (345). By the expression day's food, he understands the hundredth part of what one hundred men, "eat so as to live, labor and generate." The food is the easiest got food in the respective locality. An ounce of silver is equivalent to a day's food in Peru; the same amount in Russia is equivalent to four days' food by reason of freight and hazard of carrying it thither.
He ingeniously applies this equation for finding the value of labor-saving inventions, for expressing mathematically the ratio between demand and supply in artistic work, and claims that it can be extended further.
In the last essay on Political Arithmetic he gives us an insight into the method by which he reached this principle. "If there were but one man living in England, then the benefit of the whole territory could be but the livelihood of that one man. But if another man were added, the rent or benefit of the same would be double .... for if a man would know what any land is worth, the true and natural question must be, how many men will it feed" (252)?
Hitherto we have been confining our attention to what Petty calls intrinsic value (37). The extrinsic or accidental value is much more difficult to estimate. Roughly speaking, the extrinsic value of land might be given by taking the average of all bargains made within a definite period of time (36). An analysis of the causes which bring about the fluctuations in supply and demand should, however, be made. Why is it that rents in Ireland are lower than they are in England? He explains the difference by assigning for it the following causes (32): Rebellions, uncer-
