Thirdly, and lastly, the machines and instruments of trade, etc. which compose the fixed capital, bear this further resemblance to that part of the circulating capital which consists in money; that as every saving in the expense of erecting and supporting those machines, which does not diminish the introductive powers of labour, is an improvement of the neat revenue of the society; so every saving in the expense of collecting and supporting that part of the circulating capital which consists in money is an improvement of exactly the same kind. It is sufficiently obvious, and it has partly, too, been explained already, in what manner every saving in the expense of supporting the fixed capital is an improvement of the neat revenue of the society. The whole capital of the undertaker of every work is necessarily divided between his fixed and his circulating capital. While his whole capital remains the same, the smaller the one part, the greater must necessarily be the other. It is the circulating capital which furnishes the materials and wages of labour, and puts industry into motion. Every saving, therefore, in the expense of maintaining the fixed capital, which does not diminish the productive powers of labour, must increase the fund which puts industry into motion, and consequently the annual produce of land and labour, the real revenue of every society. The substitution of paper in the room of gold and silver money, replaces a very expensive instrument of commerce with one much less costly, and sometimes equally convenient. Circulation comes to be carried on by a new wheel, which it costs less both to erect and to maintain than the old one. But in what manner this operation is performed, and in what manner it tends to increase either the gross or the neat revenue of the society, is not altogether so obvious, and may therefore require some further explication. There are several different sorts of paper money; but the circulating notes of banks and bankers are the species which is best known, and which seems best adapted for this purpose. When the people of any particular country have such confidence in the fortune, probity and prudence of a particular banker, as to believe that he is always ready to pay upon demand such of his promissory notes as are likely to be at any time presented to him, those notes come to have the same currency as gold and silver money, from the confidence that such money can at any time be had for them.