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 upright the centre of gravity of the man G falls within the base of support; and if his load L falls without the base, as does likewise _g_, the common centre of gravity of the man and load, the consequence would be that he would fall backwards; but this is prevented, or, which is the same thing, the point _g_ is brought within the base by the man bending his body forward."[15] What is called the lay figure of the painter--a wooden image with many joints--may be bent here and there; but the artist who wanted to draw a porter with a load could not put a hundredweight upon the back of his image and keep it upon its legs. The natural machinery by which a man even lifts his hand to his head is at once so complex and so simple, so apparently easy and yet so entirely dependent upon the right adjustment of a great many contrary forces, that no automaton, or machine imitating the actions of man, could ever be made to effect this seemingly simple motion, without showing that the contrivance was imperfect,--that it was a mere imitation, and a very clumsy one. What an easy thing it appears to be for a farming man to thrash his corn with a flail; and yet what an expensive arrangement of wheels is necessary to produce the same effects with a thrashing-machine! The truth is, that the man's arm and the flail form a much more curious machine than the other machine of wheels, which does the same work; and the real question as regards the value of the two machines is, which machine in the greater degree lessens the cost of production?