We omitted to describe the beam compass, for tracing the circular lines which limit the length of divisions, on account of that apparatus being well known ; for the oppo site reason, we now proceed to give a description of the gauge, (See Fig. 4.) an implement which performs the same office in tracing parallel lines. It consists of a brass beam A, about six or eight inches long ; a socket B, of the same material, with a steel front to the left, which, sliding along the beam, may be set fast on any part of it by the finger screw S on the upper side, and a tracer of tem pered steel fixed to the end of the beam by a wedge and screw. The tracer has its point of action brought close to the inner edge, in order that it may draw a line extremely near to the edge of the scale ; its great length, as shewn in the Figure, being in no respect incommodious, is de signed for the purpose that it should be lasting. The steel front of the socket extends in the line of its action about two inches, which enables the operator to keep the beam without deviation at right angles to the edge of the scale ; and there is a projecting part in the front, of the same length, which, bearing upon the surface, keeps the tracer perpendicular ; this is discontinued in the middle, fin the purpose of admitting the tracer to be brought into contact with the front. The tracing point should be so adjusted as to be a very small quantity below the projecting part. The end of the socket to the right is on every side chamfered to a thin edge, and the beam has divided upon its different sides many lines, suited to the most common work, such as drawing the parallel lines of diagonal scales, &e. By dropping the dividing knife into these, pushing up the soc ket into contact with it, and there fastening it, the operator places the steel front and the tracer at the proper distz.nce for the performance of his work. In cases of less frequent occurrence, he clamps the socket by trial, so as to make the tracer pass tluough points previously laid down with the a tool to be described presently.
The process of dividing from straight line patterns be ing exactly similar to that from the dividing plate, it must be quite unnecessary to repeat it. Wt.- mar, however, ob serve, that in the former the errors go undiminished to the work while those of the dividing plate are contracted in the ratio of the radius of the plate and work ers pursue the co,,trary process ; they fix a plate upon the centre of their dials, al.0 transfer the divi sions outwards. It is not, however, now uncommon for them, in their very hest work, to send their dial plates to the instrument maker to be divided.
In straight line dividing, the operator has frequent oc casion to divide lines to which none of his patterns will ap ply ; but there is an expedient that he ought not to be un acquainted with, which will enable him, to a certain extent, to overcome this Suppose he were required to set off Finch inches ; that he knows the proportion they beak to English inches, which are shorter ; and that he has marked off upon the scale to be divided, according to this proportion, the total length of French inches required : he may divide it from his English inch pattern. In this case his work must be placed at an angle with his pattern. The proper inclination is readily found by trial ; and as the square must run along the pattern, its blade will not be at right angles with his work. On this account he will have
to perform two distinct operations. In the first he is to go through his work, only making slight notches in the hypo thenusal, or inclined line, with the dividing knife ; in the second, with the square applied to the edge of the instru ment which is to receive the divisions, he must, by seeing and feeling those notches, cut the corresponding perpendi cular strokes.
Suppose, again, that Spanish inches were to be divided from the same pattern : These are shorter than English inches, and will require but one operation. In this case the square must be applied to the work, and the parts taken from the inclined pattern. We might here annex the rule for finding the angle of inclination, according to the pro portion between the pattern and work to be divided, but we do not see its use. Those who handle the dividing-knife, especially in this department of the art, arc seldom versed in computation ; and as the lines upon the pattern, as well as those upon the work to be diVided, are often remote from the edge, the angular point would seldom be found upon either, and would often fall beyond the limits of the in which the work is to be performed. We have already said, that cutting divisions upon metal is laborious to the hand ; but in ivory and wood it is not so ; for in either of these a divider will keep pace with a dexterous seamstress, a division for a stitch, for any length of time. The com mon carpenter's rule is divided in this manner, and, small as is the price of the finished instrument, the dividing bears but a small proportion to it. As a farther proof of the ce lerity with which this kind of work is performed, it may be mentioned, that the writer of this part of our article being once in want of a good piece of boxwood, exactly similar to that of the common Gunter, but without slider or divi sions, the artist who provided it charged sixpence more than he would have done for the finished instrument ; and, upon being asked the reason, gave a good one : cc the little order had put him out of his common track.'' When box, or any other kind of wood, is divided upon, the bur is first well rubbed off the divisions, and then the whole surface brought to a polish with a dry rush; the sur face is next burnished by rubbing it hard both ways, in the direction of the grain of the wood, with a clean piece of old hat, which produces an agreeable gloss; and, lastly, to blacken the divisions, a mixture of powdered charcoal and linseed oil being laid on quickly, rubbed hard and cleared away, finishes the process. In ivory, the divisions are, in the tit st place, filled with a composition of lamp-black and hard tallow, or, which is rather better, of bees-wax and olive oil : when this is hard rubbed into the strokes, the whole surface should be well rushed, and then polished with chalk and water laid upon a linen rag. It has already been said, that the bur of dividing on metal should be taken of with charcoal and water ; but in brass, the surface will have a much better appearance if the finishing- stroke is given with wet blue-stone, which is a very soft slate, or the same substance that slate pencils are made of. Divided gold and silver, however, look best when they receive their finish from the charcoal. The divided surface of all the metals is improved in appearance by being rubbed with the hand, after a little oil has been applied. No other black ing is required.