Every one knows, as a piece of school-boy acquirement, that the mechanical powers are the lever, the wheel and axle, the pulley, the inclined plane, the wedge and the screw. Very few people, however, give themselves the trouble to iuquire if this classification is correct, and they may be somewhat astonished when informed that there are but two, in place of the time-honored six that have so long held an undisputed sway in the mechanical portion of all books on popular natural philosophy. First, there is the levcr ; the wheel and axle and pulley are but circular levers, and their action depends on the same principles as a straight bar having a weight, a power and a fulcrum. In both there is a weight to be raised, a power to do it, and the fulcrum is the axle, or turning point of the whole. They might well be called continuous levers. Second, we have the inclined plane, of which the wedge is but a modification, in fact, a double inclined plane, and the screw a spiral one ; their power and capabilities are calculated from nearly the same data, and the methods by which each of them attains its desired end is in principle precisely the same.