This Machine is delineated in [fig. 3] of [Plate 21]. It has several properties which I think important in the process of grinding colours, either in a wet state or a dry. It consists of a frame A B, which has a hollow centre, through which the axis of the bevel wheel C D is brought in such manner as to geer with the bevel pinion P, in whatever position the frame A B may be placed. The axis of the pinion P carries a vessel of which E F G is a section, and in which rolls a well turned and heavy ball H, upon the colour to be ground: which it crushes in the line of direction of its centre, and to a greater or lesser width according to the diameter of the ball, as compared with the section of the groove E G, in which it rolls. Now as the motion of the vessel E G F, is oblique to the perpendicular, the contact between it and the ball does not take place in any great circle of the latter: but is constantly varying by a twist in its motion dependent upon the angle of the vessel’s inclination to the horizon. From hence arises the impossibility of any colour remaining on the ball unground: and in order likewise, that none may remain uncrushed in any part of the vessel E F G, the frame A B gives it constantly new positions, one of which is represented by the dotted lines I K: where it is seen that the ball bears on a different line of the vessel’s bottom than it did before. This also adds still greater change of action to the ball itself, and occasions (taking both these properties together) an unbounded variety of effect, which necessarily brings every particle of colour under the ball by the mere continuance of motion: and thus grinds it all without any care on the part of the attendants. It may be added, that this vibrating motion of the frame A B, is easily made to result from an eccentric stud and proper connecting rods behind the frame; all which is too easy to require further description.
OF
A DYNAMOMETER,
Or a second Machine to measure power & resistance in motion.
In [Plate 21] [fig. 4], there is a representation of this Instrument. It is composed of a frame A B, containing a strong shaft C D, on which are placed the three following objects. First, a fixed pulley E, working by a strap, the Machine whose resistance is to be measured. 2ndly, a loose pulley F, receiving the power from the mover whatever it be. And 3rdly, a barrel G, which is the acting pulley, when the strap is put on it from F in the common method. But this barrel G acts by means of a barrel-spring within it, which is hooked by one end to the boss of the shaft, and the other to the rim of the barrel, as is usual for barrel-springs in general. Now the power produces the desired motion by coiling this spring to the necessary degree: and to make that degree visible, there is fixed to this barrel G a spiral s, which as the spring bends, drives outward the stud t, and with it the finger v, which, pointing to the graduated scale, shews at once the number of pounds with which the spring acts on the shaft C D to turn it. By these means the stress on the straps and on the Machine turned is known; of which also the velocity is easily determined by counting the number of revolutions performed by either of the pulleys E F G, which are alike in diameter.