A combination of two inclined planes joined at their bases is called a wedge. This simple machine is used to split wood, rocks, etc., and to raise heavy weights short distances. The power of the wedge cannot be accurately estimated, as the force, number of blows, and incline all have to be taken into account. In splitting wood (Fig. 27), the sides of the opening in the log act as levers, and thus force the mass apart in advance of the point of the wedge. More power is gained by striking the head of the wedge with either a small or a large hammer, than by pressure, as the momentum of the blow tends to shake the particles of matter and cause them to separate.
The lifting power of the wedge is utilized in dockyards, where large vessels are raised by its agency. The heads of hammers are fastened on by wedges driven in at the part of the handles near the heads. Nails, knives, needles, razors, hatchets, chisels - all act on the principle of wedges. A saw in motion represents a series of wedges which are drawn along and pressed on the object to be cut. When the edge of a razor is examined by a microscope, it is seen to be sawlike in formation; by being drawn along the beard, it cuts off the hairs.