A shaft sunk at this depth, for instance, and the machinery and stores cost £10,000, and when you get the coal to the surface that coal will pay the standard of subsistence of the labourers and leave £500 profit for capital; that is, 5 per cent. Capital will not accumulate if it gets less than 5 per cent. Labour will not be exercised if it gets less than its standard subsistence; therefore, the coal which lies farther along the seam, deeper than 5,000 feet, will be left untouched. It is not “worth while” to sink a shaft to try and get it. It is “below the Margin of Production.” [Illustration] What happens to the coal in the places where it gets nearer and nearer to the surface? Obviously, it is better worth while to sink shafts there than it is at 5,000 feet. You only want the same amount of labour for cutting the coal out, whether it is 5,000 feet below the surface or 2,000, and you want much less capital and labour in sinking the shafts and bringing the coal to the surface and getting the miners up and down. There is, therefore, a surplus. Thus with a shaft only 2,000 feet deep you need, say, only £5,000 worth of capital to get £500 worth of coal over and above the subsistence of the labourers. 5 per cent. on £5,000 is £250--so in that case there is a benefit of an extra £250 _after_ the “worth while” of Capital and Labour are satisfied. Over and above what is just the “worth while” of capital and labour for getting the coal you have in the shallower mines extra value, and that extra value gets larger and larger as the distance of the coal from the surface gets less and less. The deepest mine is on what we call “the margin of production.” It is just worth while to work it. The surplus values in all the shallower mines are called RENT. If a landlord owned the coal in quite a shallow part where it was within a thousand feet of the surface, he could say to the labourers and the owners of capital who were coming to dig it out: “The mine which is working at 5,000 feet is just worth your while. If you work here at 1,000 feet you will have a great deal more than 5 per cent. on your capital, and the subsistence of labour is just the same. All this extra amount of values, however, I must have, otherwise you shall not work my coal.” Since the Capitalists are content to accumulate capital for a return of 5 per cent. and the labourers to work for their subsistence, the extra amount is paid to the landlord. If one set of people refuse to pay it, there will always be another set of people who will be content to pay it and this extra amount or surplus is called “Economic _Rent_,” which is something, of course, much more strictly defined than, and different from, what we call Rent in ordinary conversation.