“In every state of society there is some fraction of their income which people find it worth while to keep in the form of currency; it may be a fifth, or a tenth, or a twentieth. A large command of resources in the form of currency renders their business easy and smooth, and puts them at an advantage in bargaining; but on the other hand it locks up in a barren form resources that might yield an income of gratification if invested, say, in extra furniture; or a money income, if invested in extra machinery or cattle.” A man fixes the appropriate fraction “after balancing one against another the advantages of a further ready command, and the disadvantages of putting more of his resources into a form in which they yield him no direct income or other benefit.” “Let us suppose that the inhabitants of a country, taken one with another (and including therefore all varieties of character and of occupation), find it just worth their while to keep by them on the average ready purchasing power to the extent of a tenth part of their annual income, together with a fiftieth part of their property; then the aggregate value of the currency of the country will tend to be equal to the sum of these amounts.”[22] [22] _Money, Credit, and Commerce_, I. iv. 3. Dr. Marshall shows in a footnote as follows that the above is in fact a development of the traditional way of considering the matter: “Petty thought that the money ‘sufficient for’ the nation is ‘so much as will pay half a year’s rent for all the lands of England and a quarter’s rent of the Houseing, for a week’s expense of all the people, and about a quarter of the value of all the exported commodities.’ Locke estimated that ‘one-fiftieth of wages and one-fourth of the landowner’s income and one-twentieth part of the broker’s yearly returns in ready money will be enough to drive the trade of any country.’ Cantillon (A.D. 1755), after a long and subtle study, concludes that the value needed is a ninth of the total produce of the country; or, what he takes to be the same thing, a third of the rent of the land. Adam Smith has more of the scepticism of the modern age and says: ‘it is impossible to determine the proportion,’ though ‘it has been computed by different authors at a fifth, at a tenth, at a twentieth, and at a thirtieth part of the whole value of the annual produce.’” In modern conditions the normal proportion of the circulation to this national income seems to be somewhere between a tenth and a fifteenth.