The Wealth of Nations by Part 1 Chapter 11 Page 134

which is paid for everything, and in countries where labour is equally well regarded, the money price of labour will be in proportion to that of the subsistence of the labourer. But gold and silver will naturally exchange for a greater quantity of subsistence in a rich than in a poor country, in a country which abounds with subsistence than in one which is but indifferently supplied with it. If the two countries are at a great distance, the difference may be very great; because though the metals naturally fly from the worse to the better market, yet it may be difficult to transport them in such quantities as to bring their price nearly to a level in both. If the countries are near, the difference will be smaller, and may sometimes be scarce perceptible; because in this case the transportation will be easy.