To these some of the following instruments are often added. A pair of beam compasses. A pair of perspective compasses. A pair of compasses with a micrometer screw. A rule for drawing lines, tending to a centre at a great distance. A protractor and parallel rule, such as is represented at fig. I KL, plate 2. One or more of the parallel rules represented, plate 2. A pantographer, or pentagraph. A pair of sectoral compasses, forming, at the same time, a pair of beam and calliper compasses. OF DRAWING COMPASSES. Compasses are made either of silver or brass, but with steel points. The joints should always be framed of different substances; thus, one side, or part, should be of silver or brass, and the other of steel. The difference in the texture and pores of the two metals causes the parts to adhere less together, diminishes the wear, and promotes uniformity in their motion. The truth of the work is ascertained by the smoothness and equality of the motion at the joint, for all shake and irregularity is a certain sign of imperfection. The points should be of steel, so tempered, as neither to be easily bent or blunted; not too fine and tapering, and yet mecting closely when the compasses are shut. As an instrument of art, compasses are so well known, that it would be superfluous to enumerate. their various uses; suffice it then to say, that they are used to transfer small distances, measure given spaces, and describe arches and circles. if If the arch or circle is to be described obscurely, the steel points are best adapted to the purpose; it is to be in ink or black lead, either the drawing pen, or crayon points are to be used. To use a pair of compasses. Place the thumb and middle finger of the right hand in the opposite hollows in the shanks of the compasses, then press the compasses, and the legs will open a little way; this being done, push the innermost leg with the third finger, elevating, at the same time, the furthermost, with the nail of the middle finger, till the compasses are sufficiently opened to receive the middle and third finger; they may then be extended at pleasure, by pushing the furthermost leg outwards with the middle, or pressing it inwards with the fore finger. In describing circles, or arches, set one foot of the compasses on the centre, and then roll the head of the compasses between the middle and fore finger, the other point pressing at the same time upon the paper. They should be held as upright as possible, and care should be taken not to press forcibly upon them, but rather to let them act by their own weight; the legs should never be so far extended, as to form an obtuse angle with the paper or plane, on which they are used. The ink and crayon points have a joint just under that part which fits into the compasses, by this they may be always so placed as to be set nearly perpendicular to the paper; the end of the shank of the best compasses is framed so as to form a strong spring, to bind firmly the moveable points, and prevent them from shaking. This is found to be a more effectual method than that by a screw. Fig. B, plate 1, represents a pair of the best compasses, with the plain point; c, the ink; d, the dotting; e, the crayon point. In small cases, the crayon and ink points are joined by a middle piece, with a socket at each end to receive the points, which, by this means, only oc◄ cupy one place in the case. Two additional pieces, fig. P, Q, plate 1, are often applied to these compasses; these, by lengthening the leg b, enable them to strike larger circles, or measure greater extents, than they would otherwise perform, and that without the inconveniences attending longer compasses. When compasses are furnished with this additional piece, the moveable leg has a joint, as at b, that it may be placed perpendi cular to the paper. Of the hair compasses, fig. L, plate 1. They are so named, on account of a contrivance in the shank to set them with greater accuracy than can be effec ted by the motion of the joint alone. One of the steel points is fastened near the top of the compasses, and may be moved very gradually by turning the screw n, either backwards or forwards. To use these compasses. 1. Place the leg, to which the screw is annexed, outermost; 2. Set the fixed leg on that point, from whence the extent is to be taken; 3. Open the compasses as nearly as possible to the required distance, and then make the points accurately coincide therewith by turning the screw. Of the bow compasses, fig. F, plate 1. These are a small pair, usually with a point for ink; they are used to describe small arches or circles, which they do much more conveniently than large compasses, not only on account of their size, but also from the shape of the head, which rolls with great ease between the fingers. It is, for this reason, customary to put into magazine cases of instruments, a small pair of hair compasses, fig. H, plate 1, with a head similar to the bows; these are principally used for repeating divisions of a small but equal extent, a practice that has acquired the name of stepping. Of the drawing pen and protracting pin, fig. I K, plate 1. The pen part of this instrument is used to draw straight lines; it consists of two blades with steel points fixed to a handle, the blades are so bent, that the ends of the steel points meet, and yet leave a sufficient cavity for the ink; the blades may be opened more or less by a screw, and, being properly set, will draw a line of any assigned thickness. One of the blades is framed with a joint, that the points may be separated, and thus cleaned more conveniently; a small quantity only of ink should be put at one time into the drawing pen, and this should be placed in the cavity, between the blades, by a common pen, or the feeder; the drawing pen acts better, if the feeder, or pen, by which the ink is inserted, be made to pass through the blades. To use the drawing pen, first feed it with ink, then regulate it to the thickness of the required line by the screw. In drawing lines, incline the pen a small degree, taking care, however, that the edges of both the blades touch the paper, keeping the pen close to the rule and in the same direction while operating: the blades should be wiped clean after use.* These directions are equally applicable to the ink point of the compasses, only observing, that when an arch or circle is to be described, of more than an inch radius, the point should be so bent, that the blades of the pen may be nearly perpendicular to the paper, and both of them touch it at the same time. The protracting pin k, is only a short piece of steel wire, with a very fine point, fixed at one end of the upper part of the handle of the drawing pen. It is used to mark the intersection of lines, or to set off divisions from the plotting scale, and protractor. The feeder, fig. O, plate 1, is a thin flat piece of metal; it sometimes forms one end of a cap to fit on a pencil, or it is framed at the top of the tracing point, as in the figure. It serves to place the ink between the blades of the drawing pens, or to pass beFor unequal and regular parallel lines, the double drawing pen, W, is extremely useful. EDIT. C 1 tween them when the ink does not flow freely. The tracing point, or pointrel, is a pointed piece of steel fitted to a brass handle; it is used to draw obscure lines, or to trace the out-lines of a drawing or print, when an exact copy is required, an article that will be fully explained in the course of this work; it forms the bottom part of the feeder O. Of triangular compasses. A pair of these are represented at fig. N, plate 1. They consist of a pair of compasses, to whose head a joint and socket is fitted for the reception of a third leg, which may be moved in almost every direction. These compasses, though exceedingly useful, are but little known; they are very serviceable in copying all kinds of drawings, as from two fixed points they will always ascertain the exact position of a third point. Fig. 12, plate 3, represents another kind, which has some advantages over the preceding. 1. That there are many situations so oblique, that the third point cannot be ascertained by the former, though it may by these. 2. It extends much further than the other, in proportion to its size. 3. The points are in all positions perpendicular to the paper. Of wholes and halves, fig. R, plate 1. A name given to these compasses, because that when the longer legs are opened to any given line, the shorter ones will be opened to the half of that line; being always a bisection. Fig. V, represents a new pair of very curious and portable compasses, which may be considered as a case of instruments in itself. The ink and pencil points slide into the legs by spring sockets at a; the ink, or pencil point, is readily placed, by only sliding either out of the socket, reversing it, and sliding in the plain point in its stead. Proportional compasses. These compasses are of two kinds, one plain, represented fig. A, plate 1; the other with an adjusting screw, of which there are two views, one edgeways, fig. 8, plate 3, the other in the |