back, with some increase or interest. Capital invested in railway wagons should pay itself back during the ten years that the wagons last on an average. The capital invested in any work may always be said to consist of wages or what is bought with wages. Thus the capital invested in railways really consisted of the food, clothes, and other commodities consumed by the labourers who made the railways. It is true that tools also were needed as well as the iron rails, sleepers, bricks, and other materials required for the work. But as these things had previously been made by labour, we may consider that the capital really invested in them was the wages of the labourers who had already made them. Thus, #when we go far enough back, we always find that the capital invested consisted of the maintenance of labourers#. #36. Investment of Capital.# We have two things to consider with regard to the investment of capital, #firstly, the quantity of the capital#, #and secondly, the length of time for which it is invested#. The same quantity of capital will keep more or less men at work, according as it is invested for shorter or longer periods. A man in growing potatoes only needs to wait for the result of his labour during one year on an average. If his food and clothing during one year cost thirty pounds, then capital worth thirty pounds is sufficient to keep him at work in this way. Three men cultivating potatoes will of course require three times as much capital, or ninety pounds worth; ten men will need three hundred pounds worth, and so on in proportion. But in growing vines it is necessary to wait several years after the vines are planted before they begin to bear. Suppose it to require five years waiting, then the labourer will want 5 x 30, or one hundred and fifty pounds worth of capital before he can grow vines. Three vine-growers will want 3 x 5 x 30, or four hundred and fifty pounds worth of capital; ten men, 10 x 5 x 30, or fifteen hundred pounds worth, and so on in proportion. Thus we see clearly that the capital required in any kind of industry is proportional to the number of men employed, and also to the length of time for which the capital remains locked up, or invested on the average. But there is no fixed proportion whatever between the number of labourers and the capital they require--it entirely depends upon the length of time in which the capital is turned over, that is, invested, and got back again. A poor savage manages to live on a few days' capital in hand; a potato grower on one year's capital. On a modern farm in which many durable improvements are made, the quantity of capital required is very much greater. To employ men upon a railway requires immense capital, because so much of it is sunk in a very fixed and durable form in the embankments, tunnels, stations, rails, and engines.