Though the principle of this instrument is as simple and complete as can be imagined, yet it falls sadly short of perfection in practice.
In the first place the numerous joints and castors, however carefully constructed, make it heavy to the hand in comparison with the unfettered pencil, and it requires much practice to be able to manage it so dexterously as to get those minute and gentle undulatione,neccesary not only to the beauty but to the utility of the drawing. The pencil too, constantly working upright, does not maintain that fine point which is required for distinctness of outline, and to these faults we must add inaccuracy of workmanship, however slight, the effect of wear, Inequalities of the paper, and other accidents.
The best way to test an instrument is to reverse the position of the tracer and pencil, thus producing a copy larger than the original, when the defects will of course be exaggerated in proportion.
The pantograph, however, though it cannot be depended upon for an accurate and finished copy, is of great service to the draughtsman, for by it may be marked off all the principal points through which to draw the lines of a plan with equal accuracy and infinitely greater facility than by any other means used for the purpose.
The annexed engraving represents a pantograph which possesses some advantages over that before described. In the first place, the fulcrum being in the centre, it requires but one castor, which is placed at c, and makes it work much lighter in hand than the old instrument, which has six ; besides which these six castors are frequently a source of annoyance by getting off the edge of the drawing-board and running over drawing-pins or anything else which may happen to be in the way. Secondly, the shape of this instrument allows it to move as freely when nearly closed as when opened wide, which is not the case with the other. This improved instrument consists of six bars, moving freely about each other at the six points of juncture, so arranged with regard to length that A P and T B are always parallel to each other. r is the fulcrum, furnished with a socket and screw (a), through which the centre bar can be moved, and which can be fastened down at any of the graduations on that bar. This socket, with the bar, turns upon the pin rising out of the centre of the flat weight, as shown in the diagram. Now the tracer T, the fulcrum r, and the pencil P must always be in a straight line. To produce a copy the same size as the original, the fulcrum must be in the centre, and the pencil and tracer, as in the engraving, equislistant from the centres of their respective arms, and consequently from the fulcrum. For a half sized copy the pencil must be shifted half way up the arm to p, and the fulcrum to,f, in the straight line Tf p, and so on for any required proportion, which is apparent by a glance at the diagram. The rule
Laid down in regard to the other instrument holds good for this, the copy bearing the same proportion to the original as the distance of the pencil from the fulcrum does to that of the tracer.
In using the pantograph it is frequently neceroary to copy the draw ing in divisions, on account of the instrument not being large enough to extend over the whole surface at once. In this case the greatest care is requisite to join the lines of one division accurately to those of the adjoining division. The best way of effecting this is to hold the tracer down upon a point in the original by one hand, moving the fulcrum about with the other, until the pencil exactly coincides with the corresponding point in the copy. To ensure accuracy, this should be tried with three or more points, as the least deviation will throw the whole plan out of position.
In practice a largo number of plans are copied of the same size as the original. The quickest method, which, however, requires great care, is to pin down a piece of tracing-paper upon the original, on which the lines are marked with a fine-pointed pencil. A piece of .paper rubbed over with blacklead ie then placed upon the paper intended for the copy, with the black side downwards, and the tracing-paper is pinned down upon it; the lines then are carefully traced over with a steel point, and the pressure transfers the blacklead to the drawing-paper beneath. The black paper is sometimes omitted, and the lines, traced by slight indentations only, are finished by hand, in the usual manner.
If it is required to reduce a plan, and the draughtsman does not possess a pantograph, the usual way is to divide the original and the paper for the copy, whatever the relative size, into the same number of squares, which will of course then bear the Rime proportion to each other as the surfaces of which they are divisions. The lines must then be set in singly, either by the assistance of the proportional com pass or scale, or by a geometrical scale drawn for the occasion. The following method is dependent only upon the compass and T square.
Draw a triangle in which A n is the base, and make A c bear the same proportion to is n as you wish the copy to bear to the original ; take any distance on the original, and set it off from n on n c, draw a line parallel to A B, and that portion of A c cut off by such parallel wi beat the required proportion to the distance set off on B c, as may be seen by the divisions 1 2 3 4 on the two sides of the triangle in the diagram. The triangle should be drawn in ink, and the distances, being marked in pencil, may be rubbed out when transferred to the paper. [EIDOGRAPH ; PROPORTIONAL COMPASSES.]