THREE:To follow this matter further. It will be found in such machines as are directed mainly to augmenting force or increasing the amount of power that may be applied in any operation, such as sawing wood or stone, the effect produced when compared to hand labour is nearly as the difference in the amount of power applied; and the saving that such machines effect is generally in the same proportion. A machine that can expend ten horse-power in performing a certain kind of work, will save ten times as much as a machine directed to the same purpose expending but one horse-power; this of course applies to machines for the performance of the coarser kinds of work, and employed to supplant mere physical effort. In other machines of application, such as are directed mainly to guidance, or speed of action, such as sewing machines, dove-tailing machines, gear-cutting machines, and so on, there is no relation whatever between the increased [59] effect that may be produced and the amount of power expended.By placing the axes of tangent gearing so that the threads or teeth of the pinions are parallel to the face of the driven teeth, [53] as in the planing machines of Messrs Wm. Sellers & Co., the conditions of operation are changed, and an interesting problem arises. The progressive or forward movement of the pinion teeth may be equal to the sliding movement between the surfaces; and an equally novel result is, that the sliding action is distributed over the whole breadth of the driven teeth.
