ii8 POLITICAL ECONOMY 



the excess of annual value of that acre over the annual value of 

 the poorest land which tenants think it worth while to cultivate. 

 We may classify all land according to the total return which it 

 will yield per acre upon capital invested in its cultivation ; and 

 we may draw a supply curve of land such that the ordinates will 

 be the total quantities of land which will return each successive 

 percentage on the capital required to cultivate it. The supply 

 diminishes as the rate of percentage increases, that is to say, 

 there is less land which will return ten per cent, on the capital 

 than will return five per cent., and still less land which will 

 return twenty or thirty per cent. 



If, therefore, tenants as a body, considered as capitalists, will 

 not cultivate any land which does not yield twenty per cent., 

 there will be far less land in the market than if they will be 

 just satisfied with ten per cent. 



Again, all tenants are not of one mind, and we may construct 

 a demand curve in which the ordinates are the total quantities of 

 land which would be let, if the land paying no rent be fixed at 

 each successive percentage. The actual quantity of land let will 

 be determined by the intersection of the two curves, and is repre- 

 sented by the height M D, Fig. 4. 



If we now build a solid on the base D'D N, such that its height 

 all along each ordinate x is the number of hundreds of pounds 

 of capital per acre required to give the percentage corresponding 

 to the length #, then we shall have a volume standing on O D'D N, 

 the contents of which will measure the total annual returns from 

 all the land cultivated. 1 The rent is the volume standing on 

 M D N, the profit received by the farmers is the volume standing 

 on O D' D M, and this is in excess of what would have just 

 tempted them to cultivate by the volume on M D P. We may, 

 therefore, considering the farmer as a capitalist and a trader, call 

 the volume on M D P his trade profit, and the volume on D' D 

 the interest on his capital. 



The effect of any tax on the land is to reduce the interest 

 which each class of land is capable of returning on the capital 



1 If 150?. per acre are required to give the percentage x of any ono class 

 of goods, the height of the ordinate perpendicular to the plane of D ' D N 

 will be 1-5. 



