PANTHEON. [Rome, in Geo°. Div.] l'ANTOCItAl'H, sometimes improperly called PENTAORAPH, is an instrument employed in copying maps, plans, and other drawings.
The annexed diagram represents that most commonly in use.
It is composed of four brass rules or bars, jointed to each other at the points e D e r. Three joints require to be carefully executed, as upon the smoothness and steadiness of their motion the accuracy of the instrument principally depends. In those of the beat construction there is a small metal cock screwed to tho bar of mcli joint, which support,' the upper end of the axis upon which it turns. This Is a material improvement upon the conunon flat compass joint, as, beeides lengthening the axis, it gives additional strength to the bar where it most requires it. A joint of this construction is shown in fig. 2.
At the point c is fixed a small tube which carries the tracing point, .o fitted as to move freely within it, but without shaking; fig. 3 repre sent. the tracer within the tube, The bar E D and the lower part of the bar an are furnished each with a tube, jig. 4, similar to that at c, but moveable on the bar, with a similar screw to fasten it down at any required point. Fig. 5 is the pencil stem, which fits either of the tubes In the same manner as the truer; on the top of the stem is a cup to receive a email weight to keep it down upon the paper, and the lower ered carries a pencil, or, which Is better, a " lilonLues paint." A elk cord is attached to the pencil stem, carried through eyes made for the purpose, over the joint. Z a v, one of which is shown on the top of fig. 2, and fixed in a notch in the top of the tracer, Ye. 3, so that the pressure of the thumb upon the cord lift. the pencil from the paper when required. Fib. 6 is a flat leaden weight with a braes stem rising out of it, which fits in the tubes in the same manner as the pencil and treeing point ; this is called the fulcrum, as being the point upon which, the whole instrument moves; the weight has three or four line points on it. under surface to keep it from shifting upon the paper. The whole instrument is supported by six revolving feet or castor., which should move very freely In all directions. One of these cantors is shown wider the joint,,ifo. 2. l'ho pin or fulcrum is placed near the edge of the weight, so as to allow room for the castor to work when the fulcrum is near the points A or D.
The proportion of the bars is of no consequence, so that E n is equal to s r and Z a to r D, in which case they will always form a pamllel ogram, by which name the instrument is sometimes called. Now if the tracer at c is carried over the lines of the plan, the fulcrum-tube being fixed at A, and the pencil-tube at 0, the pencil will make an exact copy of every line, but only half the size of the original, which may be proved in the following manner : The points A, o, and c, are capable of being brought close together, and when the instrument is open, as in the figure, o is exactly half way between o and A; c then travels twice as quickly as o in the direction A 0 0, so that to whatever extent the pantograph may be opened, o and o being considered as points in a lever of which A is the fulcrum, it will be seen that if c is made to describe an arc of a circle of any radius, 0 at the same time is describing an aro of a circle of half the radius, so that c moves in a direction perpendicular to an o twice as fast as o. Now it was shown above that it moved twice as fast as o in the direction a o c, and as by the composition of these two motions, all lines, whether rectilinear or curved, are produced, it follows that the pencil at 0 will produce a copy half the size of the original traced over by the point at o, as before stated.
To produce a copy one-fourth the size of the original, we must shift the pencil from o to y, and the fulcrum train a to a, a g being one fourth the length of a c, and so on for all other proportions less than half, the constant rule being—" As the distance of the pencil from the fulcrum is to the distance of the tracer from the fulcrum, so will be the size of the copy to that of the original." No calculation is required in practice, as the arms A E and n are graduated for all proportions from one-twelfth to eleven-twelfths inclusive. If it be required to produce a copy inert than half the size of the original, the fulcrum must be placed on the arm E D, and the pencil on a E, the rule above given still holding good, so that for a copy the same size as the original the fulcrum must be placed at o, and the pencil at a. It must be borne in mind, however, that when the fulcrum is on the arm E D, the copy will be inverted.