In [Plate 3], [Fig. 1] and [3], M M, represent two cheeks, standing parallel to each other, and forming a cage or frame by means of the cross bars E and the nuts F G. A P, [Fig. 2], is the principal axis of the Dynamometer, fixed to the wheel R N of which it is the centre of motion. It has a square end A, formed to receive the wheels and other supplemental parts, to be mentioned below. After the square A, comes a bearing E, to fit the steps in the frame; and beyond the wheel R N is a cylindrical part O, fitted to the hollow axis T of the wheel or frame I K, ([Fig. 4]); and in fine the form P of this shaft fits and turns in the cannon of the axis B H, of the wheel C D; so as, when put together and connected with the frame I K, to assume the form C R F G of the third figure. L P, [Fig. 3] and [4], are two intermediate wheels (thus placed to balance each other on the common centre T) whose axes turn on proper steps in the frame I K; and which by their teeth connect the motion of this frame with that of both the wheels R N, and C D.
Such are the parts of the Dynamometer properly so called; and they are shewn as in their places in [Plate 1], where the parts above described, as far as visible, are marked with the same letters. Moreover, this figure shews a scale-bason P, to receive the weights used to measure equable powers, as will be seen hereafter.
[Plate 4] contains some of the auxiliary parts of this Machine. But before we proceed to describe them, it may be proper to observe that the measuring power, by the action of which at K, ([Plate 1]) the energy of the force is transmitted to the resistance, must, to meet every case, be susceptible of change, according as the resistance or force to be measured is uniform or convulsive. For example, in a mill grinding corn, driven by a fall of water, the whole process is sensibly uniform, and a weight at P is the proper measurer. But if it were desired to measure the effect of a pump driven by water, or of a tilt hammer worked by a Steam Engine, then the measuring power at P must be a spring: for in these cases the vis inertiæ of a weight would add to its force of gravity when suddenly raised, or detract from it when the resistance should suddenly give way. Whenever therefore, the force and resistance are both equable, a weight will best measure them; and when either is convulsive, a spring: but a spring so equalized as to offer the same resistance at every degree of tension it may have to sustain.
In the [6th.] and [7th. Figures], ([Plate 4]) these demands are fulfilled. The [first] represents a barrel-spring, similar to that of a watch, but surrounded by a fusee, the increasing radii of which compensate for the increased tension of the spring in the barrel G; so that the action of the system on the chain is always the same.