The pulley or tackle is of such general utility, that it would seem unnecessary to point out any particular instance.
Of the inclined Plane, and of the Wedge.
The inclined plane has in its effects a near analogy to the lever; and the forces by which the same weight tends downwards in the directions of various planes, will be as the sines of their inclinations.
The wedge is composed of two inclined planes joined together at their common bases, in the direction of which the power is impressed.
This instrument is generally used in splitting wood, and was formerly applied in engines for stamping watch plates. The force impressed is commonly a blow, which is found to be much more effectual than a weight or pressure. This may be accounted for on the principles which obtain when resisting bodies are penetrated, as if the mass and velocity vary, the depths to which the impinging body penetrates will be in the compound ratio of the masses and the squares of the velocities.
All cutting instruments may be referred to the wedge. A chizel, or an axe, is a simple wedge; a saw is a number of chizels fixed in a line: a knife may be considered as a simple wedge, when employed in splitting; but if attention be paid to the edge, it is found to be a fine saw, as is evident from the much greater effect all knives produce by a drawing stroke, than what would have followed from a direct action of the edge.
Of the Screw, and of mechanical Engines, in general.
The screw is composed of two parts, one of which is called the screw, and consists of a spiral protuberance, called the thread, which is wound round a cylinder; and the other called the nut, is perforated to the dimensions of the cylinder, and in the internal cavity is cut a spiral groove adapted to receive the thread.
It would be difficult to enumerate the very many uses to which the screw is applied. It is extremely serviceable in compressing bodies together, as paper, linen, &c. It is the principal organ in all stamping instruments for striking coins, or making impressions on paper, linen, or cards, and is of vast utility to the philosopher, by affording an easy method of measuring or subdividing small spaces. A very ordinary screw will divide an inch into 5,000 parts; but the fine hardened steel screws, that are applied to astronomical instruments, will go much farther.
It is easy to conceive, that when forces applied to mechanical instruments are in equilibrium, if the least addition be made to one of them, it will preponderate and overcome the effort. But the want of a perfect polish or smoothness in the parts of all instruments, and the rigidity of all ropes, which increases with the tension, are great impediments to motion, and in compounded engines are found to diminish about one fourth of the effect of the power.