It has already been said, that the general and well-known agents usually regarded as the principal sources of mechanical powers are gravity, acting on solid bodies, the fall of water, which is but gravity acting on fluids, air, heat, and animal strength. For the useful direction and application of the first four of these, that is, of all of them which belong to inanimate nature, some intermediate apparatus or contrivance becomes necessary; and this apparatus, whatever its form, is a machine. A machine is an invention for the application of motion, either by changing the direction of the moving power, or by rendering a body in motion capable of communicating a motion greater or less than its own to other bodies, or by enabling it to overcome a power of greater intensity or force than its own. And it is usually said that every machine, however apparently complex, is capable of being resolved into some one or more of those single machines, of which, according to one mode of description, there are six, and according to another, three, called the mechanical powers. But because machinery, or all mechanical contrivance, is thus capable of resolution into a few elementary forms, it is not to be inferred that science, or art, or both together, though pressed with the utmost force of human genius, and cultivated by the last degree of human assiduity, will ever exhaust the combinations into which these elementary forms may be thrown. An indefinite, though not an infinite, reach of invention may be expected; but indefinite, also, if not infinite, are the possible combinations of elementary principles. The field, then, is vast and unbounded. We know not to what yet unthought of heights the power of man over the agencies of nature may be carried. We only know that the last half-century has witnessed an amazingly accelerated progress in useful discoveries, and that, at the present moment, science and art are acting together with a new companionship, and with the most happy and striking results. The history of mechanical philosophy is, of itself, a very interesting subject, and will doubtless be treated in this place fully and methodically, by stated lecturers.