All such questions must be determined by the cost of production; and that cost in agriculture is made up of the rent of land, the profit of capital, and the wages of labour--or the portions of the produce belonging to the landlord, the farmer, and the labourer. Where rent is high, as in the immediate neighbourhood of large towns, it is important to have the labour performed as carefully as possible, and the succession of crops stimulated to the utmost extent by manure and labour. It is economy to turn the soil to the greatest account, and the land is cultivated as a garden. Where rent is low, it is important to have the labour performed upon other principles, because one acre cultivated by hand may cost more than two cultivated by the plough; and though it may be the truest policy to carry the productiveness of the earth as far as possible, field cultivation and garden cultivation must have essential differences. In one case, the machine called a spade is used; in the other, the machine called a plough is employed. The use of the one or the other belongs to practical agriculture, and is a question only of relative cost. [Illustration: Centre of gravity] And this brings us to the great _principle_ of all machinery. A tool of the simplest construction is a machine; a machine of the most curious construction is only a complicated tool. There are many cases in the arts, and there may be cases in agriculture, in which the human arm and hand, with or without a tool, may do work that no machine can so well perform. There are processes in polishing, and there is a process in copper-plate printing, in which no substance has been found to stand in the place of the human hand. And, if therefore the man with a spade alone does a certain agricultural work more completely than a man guiding a plough, and a team of horses dragging it (which we do not affirm or deny), the only reason for this is, that the man with the spade is a better machine than the man with the plough and the horses. The most stupid man that ever existed is, beyond all comparison, a machine more cunningly made by the hands of his Creator, more perfect in all his several parts, and with all his parts more exquisitely adapted to the regulated movement of the whole body, less liable to accidents, and less injured by wear and tear, than the most beautiful machine that ever was, or ever will be, invented. There is no possibility of supplying in many cases a substitute for the simplest movements of a man's body, by the most complicated movements of the most ingenious machinery. The laws of mechanism are the same whether applied to a man, or to a lever, or a wheel; but the man has more pliability than any combination of wheels and levers. "When a porter carries a burden, the attitude of the body must accommodate itself to the position of the common centre of gravity of himself and his load. Thus, in the accompanying figures it will be observed that when the man stands