[Fig. 9] of this [Plate] shews a real steel-yard made on this principle; the power of which, under its present length, is as 40 to 1. In this Machine all the centres are fixed: and the load is suspended on knife-edges, the distances of which from each other and from the common centres are invariable—as they must be in all instruments of this nature.
OF
A RETROGRAPH,
Or a Machine to write backwards, for Engravers.
This Machine is exhibited in the two [figures 10 and 11] of [Plate 19]. It is composed of a straight ruler A B, having an exactly dove-tailed mortice made along it, to receive the rollers, (or slides) by means of which the parallelogram C D E F slides up and down in this mortice. This parallelogram is composed of four rulers C D, D E, E F, and F C, connected by cannons or tubes fixed to every-other arm: and on which the contiguous rulers turn very correctly. Through which moreover, in two cases, F D the drawing pencils are introduced, and under which in other two cases, C and E, the guide rollers already mentioned are nicely fixed by the screws on which they turn. This is seen by an elevation in [fig. 10], where p marks one of these rollers, and o q the end of the ruler supposed fixed to the paper by proper blunt points, &c. At r is seen one of the tubes which form the joints C and E: and r t, are, one the writing pen, and one the retrographic style or pencil. [Fig. 11] is a plan of the whole Machine: where if the hand guiding the pen D goes upward, the tracer F rises too. But if the pen or hand D moves to the right, the tracer moves to the left at the same moment. In a word this is to write backward in the sense of engravers, who thus write that their letters may proceed forward after one impression.
If it were desirable to give the engraver the same facility he has in the use of a pen, the tracer t, [fig. 10], would be terminated above as a hollow conical cup, into which he would introduce a pointed style held as a pen. In this case the tracer t, would be made as short or low as possible, to bring the style so much the nearer to the paper; and thus to prevent all anomalous movements.