We state this principle broadly, in our examination into the value of machinery in diminishing the cost of producing human food. A machine is not perfect because it is made of wheels or cylinders, employs the power of the screw or the lever, is driven by wind or water or steam, but because it best assists the labour of man, by calling into action some power which he does not possess in himself. If we could imagine a man entirely dispossessed of this power, we should see the feeblest of animal beings. He has no tools which are a part of himself, to build houses like the beaver, or cells like the bee. He has not even learnt from nature to build, instinctively, by certain and unchangeable rules. His power is in his mind; and that mind teaches him to subject all the physical world to his dominion, by availing himself of the forces which nature has spread around him. To act upon material objects he arms his weakness with tools and with machines. As we have before said, tools and machines are in principle the same. When we strike a nail upon the head with a hammer, we avail ourselves of a power which we find in nature--the effect produced by the concussion of two bodies; when we employ a water-wheel to beat out a lump of iron with a much larger hammer, we still avail ourselves of the same power. There is no difference in the nature of the instruments, although we call the one a tool, and the other a machine. Neither the tool nor the machine has any force of itself. In one case the force is in the arm, in the other in the weight of water which turns the wheel. The distinctions which have been taken between a tool and a machine are really so trivial, and the line of separation between the one and the other is so slight, that we can only speak of both as common instruments for adding to the efficiency of labour. The simplest application of a principle of mechanics to an every-day hand-tool may convert it into what is called a machine. Take a three-pronged fork--one of the universal tools;--fasten a rope to the end of the handle; put a log under the fork as a fulcrum; and we have a lever, when pulled down by the rope, which will grub up a strongly-rooted large shrub in a few minutes. The labourer has called in a powerful ally. The tool has become a machine. [Illustration: A tool made a machine]