Closely related to Franklin's political pamphlets are his writings on economics, which, though undertaken with a political or practical purpose and not in a purely scientific spirit, rank him as the first American economist. He wrote in 1729 _A Modest Enquiry into the Nature and Necessity of a Paper Currency_, which argued that a plentiful currency will make rates of interest low and will promote immigration and home manufactures, and which did much to secure the further issue of paper money in Pennsylvania. After the British Act of 1750 forbidding the erection or the operating of iron or steel mills in the colonies, Franklin wrote _Observations concerning the Increase of Mankind and the Peopling of Countries_ (1751); its thesis was that manufactures come to be common only with a high degree of social development and with great density of population, and that Great Britain need not, therefore, fear the industrial competition of the colonies, but it is better known for the estimate (adopted by Adam Smith) that the population of the colonies would double every quarter-century; and for the likeness to Malthus's[12] "preventive check" of its statement: "The greater the common fashionable expense of any rank of people the more cautious they are of marriage." His _Positions to be examined concerning National Wealth_ (1769) shows that he was greatly influenced by the French physiocrats after his visit to France in 1767. His _Wail of a Protected Manufacturer_ voices a protest against protection as raising the cost of living; and he held that free trade was based on a natural right. He knew Kames, Hume and Adam Smith, and corresponded with Mirabeau, "the friend of Man." Some of the more important of his economic theses, as summarized by W. A. Wetzel, are: that money as coin may have more than its bullion value; that natural interest is determined by the rent of land valued at the sum of money loaned--an anticipation of Turgot; that high wages are not inconsistent with a large foreign trade; that the value of an article is determined by the amount of labour necessary to produce the food consumed in making the article; that manufactures are advantageous but agriculture only is truly productive; and that when practicable (as he did not think it practicable at the end of the War of Independence) state revenue should be raised by direct tax.