In [fig. 1 and 2], (see [Plate 28]), A B are two horizontal planes of hard wood or metal, connected, at a proper distance, by the pillars C D, shewn in [fig. 1] only. E F are two Sectors of a large cylinder, united at the point a, either by a good hinge or by a joint composed of a hollow prism fixed to the upper sector E, and of a solid one, more acute, fixed to the lower sector F; so that, in the latter case, this joint works with an insensible degree of friction, and thus occasions a great saving of power.
In the working of this Press, the joint just mentioned, however made, describes a straight line, parallel both to the floor B G and the ceiling H A, which have been already shewn to be parallel to each other: and thus are the joint a and the sectors E F suspended to the cap or ceiling A H by a pair of triangular braces I a K, which slide smoothly in two dove-tailed grooves A m. Moreover, to the lower sector F are fixed two working arcs b c, one on each side of the Press, and whose radii are exactly equal to that of the upper sector E (whose circumference, therefore, is invisible in [fig. 1].) Further, just above these arcs, and in the middle of the slide I K, are placed, on proper centres, a pair of grooved pulleys P, destined to work the under sector, without disturbing the motion of the upper one, which latter is a rolling motion under the aforesaid ceiling A H. For the said purpose, a metallic cord or chain is fixed at m ([fig. 1]), which, passing round one of the pulleys P, is led to the end n of the arc b c, n o; and near A is fixed a similar cord, which, carried round the other pulley at P, is led to the angle o of the same arc b c, n o. By these means, the sector F is fixed both in place and position, as long as the slide I K retains it’s present position and state. But, again, a system of similar cords, placed under the ceiling A H, near the edges of the upper sector E, determines the place of that sector, in every case, except a change of position; for a rolling motion can still have place, without occasioning any other change.
When, therefore, a pulling bar, a crank and fly, or any other prime mover, applied at the joint a, carries that joint (say) toward the pillar D, that motion takes place without any rubbing of surface either above or below; for, when the upper section has rolled under the ceiling A H, into the position n p q, the lower section has rolled upon the plate s t, into the position q r s: in such sort that the analogous angles o t, p r of both sectors are always found in the same perpendicular line—or plane—o t, p r; the cause of which I shall now endeavour to unfold.
When a wheel, in general, rolls on or against any fixed plane (and the cords m P, A P, now act the part of a fixed plane), the point of it’s circumference the most distant from that plane, moves, in a direction parallel to it, just twice as fast as the centre of such wheel, because it is twice as far from that plane, the virtual centre of its motion: (an example of which is found in the wheel of a carriage, whose top moves forward just twice as fast as it’s axle-tree.) Supposing, then, in the present case, the frame I a K, with the pulleys P to glide toward the right hand, the cord A o fixed near A, will turn the arc b c to the right, twice as fast as the centre of the pulley P moves in that direction: and if this impulse had acted on the joint a, while fixed in position, the arc b c would have turned too much by half. But it so happens (if this expression may be used), that the joint a itself moves in that direction once as fast as the pulley-pin; so, that the motion remaining to the sector F is a single motion, merely sufficient to keep the two sectors E and F directly under each other, or within the same perpendicular lines p r, n q s, &c.
Thus, it appears, that the turning motion of the two sectors is the same; and that a given point of the lower one will always visit the same point of the corresponding plane s t, independently of contact with any substance lying on it; and that, therefore, the pressure, though successive, is perpendicular, having no tendency to displace or pucker the paper laid on it; besides which, it may be observed, that the power of this Press is immense, from the length of the radii of the sectors E F, and the absence of any rubbing motion.
I observe, further, that racks, made with teeth on my principle, either singly inclined with cheeks, as in [Plate 14], or with teeth in the V form, will produce a more certain effect than the cords and pulleys above described, provided the arcs b c, and the upper sector E, be prepared and toothed accordingly.