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tracer from one part of the original to another, and thus to prevent false lines being made on the copy. The pencil holder is surmounted by a cup, into which sand or shot may be put, to press the pencil more heavily on the paper, when found necessary.
If the object were to enlarge the map to double its scale, then the tracer must be placed on the arm DF, and the pencil at C; and, if a copy were required of the same scale as the original, then, the sliding indices still remaining at the same divisions on DF and A B, the fulcrum must take the middle station, and the pencil and tracing point those on the exterior bars A B, AC of the instrument.
Though the pantagraph affords the most rapid means of reducing a map or drawing, we cannot recommend its use for enlarging a copy, or even for copying on the same scale, especially when the original drawing is a complicated one. The eidograph described in Heather's “ Surveying and Astronomical Instruments,” in Weale’s Series, should be employed for that purpose, or one or other of the following methods.
To produce a copy of the same size as the original.-Lay & sheet of tracing paper, having its under side rubbed over with powdered black lead, upon the paper intended to receive the copy; the original being then placed over both, the whole may be made to lay steadily by weights placed thereon: the tracing point may now be carefully passed over all the lines of the drawing, with a pressure proportionate to the thickness of the paper; and the paper beneath will receive corresponding marks, forming an exact copy, which is afterwards to be inked in.
NOTE. Copies of maps are also taken for ordinary purposes by laying tracing paper thereon, through which, from its almost transparent thinness, all the lines of the original can be seen, and readily traced with ink, on the tracing paper. Copies, thus obtained, are called tracings.
Another method.—The drawing or map is placed on a large sheet of plate-glass, called a copying glass, and the paper to receive the copy placed over the drawing. The glass is then fixed in such a position as to have a strong light to fall upon it from behind, and to shine through it and both the original drawing and the paper to receive the copy. By this means the lines of the original drawing become visible through the paper to receive the copy, which can be made with precision and ease, without any risk of soiling or injuring the original.
To copy with exactness on a reduced or enlarged scale.—For this purpose we have recourse to the method of squares, by which the most minute details may be copied with accuracy. This perhaps may be best shown by an example. Let figure 1, in the annexed engraving represent a plan of an estate, which
it is required to copy upon a reduced scale of one half. The copy will therefore be half the length and half the breadth, and consequently will occupy but one fourth the space of the ori ginal. The subject is the map of an estate, but the process would be precisely the same, if it were an architectural, me chanical, or any other drawing. Fig. 1.
Fig. 2. 6 d f g h 1
f 9 Draw the lines FI, FG at right angles to each other; from the point F towards I and G, set off any number of equal parts, as Fa, ab, bc, &c., on the line FI, and Fi, ik, kl, &c., on the line FG: from the points in the line FI, draw lines parallel to the other line F G, as a a, b b, cc, &c., and from the points on F G, draw lines parallel to FI, as ii, k k, ll, &c., which being sufficiently extended towards I and G, the whole of the original drawing will be covered with a reticule of small but equal squares. Next draw upon the paper intended for the copy, a similar set of squares, but having each side only onehalf the length of the former, as is represented in figure 2. It will now be evident that if the lines AB, BC, CD, &c., figure 1, be drawn in the corresponding squares in figure 2, a correct copy of the original will be produced, and of half the original scale. Commencing then at A, observe where in the original the angle A falls, which is towards the bottom of the square, marked de. In the corresponding square, therefore, of the copy, and in the same proportion towards the left-hand side of it, place the same point in the copy: from thence tracing where the curved line A F crosses the bottom line of that square, which crossing is about two-fifths of the width of the square from the left hand corner towards the right, and cross it similarly in the copy. Again, as it crosses the right hand bottom correr in the second square below de, describe it so in the
copy; find the position of the points similarly where it crosses the lines ff and gg, above the line ll, by comparing the distances of such crossings from the nearest corner of a square in the original, and similarly marking the required crossings on the corresponding lines on the copy. Lastly, determine the place of the point B, in the third square below gh on the top line; and a line drawn from A in the copy, through these several points to B, will be a correct reduced copy of the original line. Proceed in like manner with every other line on the plan, and its various details, and you will have the plot or drawing laid down to a small scale, yet bearing all the proportions in itself exactly as the original.
It may appear almost superfluous to remark, that the process of enlarging drawings, by means of squares, is a similar operation to the above, excepting that the points are to be determined on the smaller squares of the original, and transferred to the larger squares of the copy. The process of enlarging, under any circumstances, does not, however, admit of the same accu. racy as reducing.
THE PRISMATIC COMPASS. With this instrument horizontal angles can be observed with great rapidity, and, when used with a tripod stand, with a considerable degree of accuracy; it is, therefore, a useful instrument for filling in the details of an extensive survey, after the principal points have been laid down by means of observations made with the theodolite, hereafter to be described. It was used for this purpose by the gentlemen engaged in making the Ordnance surveys.
C is a compass card, di vided usually to every 20, or third
part of a degree, and having attached to its under side a mag. netic needle; n is a spring, which, being touched by the finger, acts upon the card and checks its vibrations, so as to bring it sooner to rest, when making an observation.
S is the sight vane, having a fine thread stretched along its opening, which is to cut the point to be observed by the instrument. The sight vane is mounted upon a hinge joint, so that it can be turned down flat in the box, when not in use. P is the prism, attached to a plate sliding in a socket, and thus admitting of being raised or lowered at pleasure, and also supplied with a hinge joint, so that it can also be turned down into the box, when not in use.
In the plate to which the prism is attached, and which projects beyond the prism, is a narrow slit, forming the sight through which the vision is directed, when making an observation. On looking through the slit, and raising or lowering the prism in its socket, distinct vision of the divisions on the compass card, immediately under the sight-vane, is soon obtained; and these divisions, seen through the prism, all appear, as each is successively brought into coincidence with the thread of the sight-vane by turning the instrument round, as continuations of the thread, which is seen distinctly through the part of the slit that projects beyond the prism.
The method of using the instrument is as follows :—the sight-vane S, and the prism P, being turned up on their hinge joints, as represented in the figure, hold the instrument as nearly in a horizontal position as you can judge, or, if a tripod stand be used, set it as nearly as you can in a horizontal posi. tion by moving the legs of the stand, that thence the card may play freely. Raise the prism in its socket till the divisions on the card are seen distinctly through it, and, turning the instrument round, until the object to be observed is seen through the portion of the slit projecting beyond the prism, in exact coincidence with the thread of the sight-vane, bring the card to rest by touching the spring n; and then reading at the division upon the card, which appears in coincidence with the prolongation of the thread, gives the magnetic azimuth or bearing of the object observed, or the angle which a straight line, drawn from the eye to the object, makes with the magnetic meridian.* The magnetic azimuth of a second object being
* The magnetic meridian now makes an angle of 22° with the true meridian at London, the north point of the compass being 22° west of the true north point. This angle is called the variation of the compass, and is different at different places, and also at the same place at different times. Since this varia. tion will affect equally, or nearly so, all azimuths observed within a limited extent, and during a limited time, the angles subtended by any two of the
obtained in the same manner, the difference between these two azimuths is the angle subtended by the objects at the place of the eye, and is quite independent of the error in the azimuths, arising from the slit in the prism not being diametrically opposite to the thread of the sight-vane.
For the purpose of taking the bearings of objects much above or below the level of the observer, a mirror R is supplied with the instrument, which slides on and off the sight-vane S, with sufficient friction to remain at any part of the vane that may be desired. It can be put with its face either upwards or
downwards, so as to reflect the images of objects considerably 9 above or below the horizontal plane of the eye of the observer.
If the instrument be used for obtaining the magnetic azimuth of the sun, the dark glasses D must be interposed between the sun's image and the eye.
There is a stop in the side of the box, not shewn in the figure, by touching which a little lever is raised and the card thrown off its centre; as it always should be, when not in use, or the constant playing of the needle would wear the fine agate point, on which it is balanced, and the sensibility of the instrument would be thereby impaired. The sight-vane and prism being turned down, à cover fits on the box, which is about three inches diameter, and one deep; and the whole being packed in a leather-case, may be carried in the pocket without inconvenience.
THE BOX SEXTANT, This instrument, which is equally portable with the prismatic compass, forming, when shut up, a box about three inches in diameter, and an inch and a half deep, will measure the
objects observed, being the difference of their azimuths, will not be affected by the variation; and hence the map or plan may be constructed with all the objects in their proper relative positions; but the true meridian must be first laid down on the map, if required, by making allowance for the variation.