The bur is generally taken off the lines with a tool call ed a scraper, and then cleared with the knife. After this the surface is rubbed with charcoal (that from the willow is best) and water, which leaves the surface smooth, with out producing a gloss that would be unpleasant to the eye.
Having been thus minute in the description of the tools, that of their application will be short indeed.
A compass ring, for instance, as before mentioned, is at tached to the dividing plate, for the purpose of being gra duated ; its zero, or N, being placed so, that the index, when set to it, may agree also with the zero of the plate. In this position of things, the operator must drop the point of the dividing knife into the line on the plate, and, press ing the index to prevent its moving, cut the corresponding stroke upon the ring. He must be careful to hold the knife steady, and in exactly the same position, while he sets and cuts. His eye must be directed to the left side of the knife, in order that he may see that he holds it in con tact with, and parallel to, the edge of the index. The in dex is now drawn forwards something more than the value of a division, and the knife being fixed in a second line, it is then pushed back into contact with it, and a second stroke cut as before ; thus proceeding from right to left until the circle is completed. In dividing upon metals, this work is laborious for the hands, which will require lt.equent intervals of rest : during these the divider should examine his work with a magnifying glass. In cases where the hand has not power to cut the strokes deep enough at once, which does not unfrequently happen, in stead of cutting twice, the operator will proceed with more ease and expedition, if he goes round by single cutting, and afterwards completes his work : for, in the latter part, he will not have occasion for either the dividing plate or the index ; the knife, if well set, will follow the former strokes with accuracy and neatness. The bur must be worn off the divisions by rubbing the surface with charcoal and wa ter, as was before done with the circular lines ; but the scraper must not be employed.
Common dividing, as it applies to straight lines, is so si milar to circular dividing, that little need be said about it.
Figs. 3. and 4. represent the necessary apparatus; the di viding knife is common to both. AA is the pattern, and B rs a scale to be divided into inches and tenths CCC re presents what is called the dividing square. This tool consists of a thin tempered piece of steel, extending, in the line of the pattern, about six inches, and at right an gles somewhat more, and a stock about two-tenths of an inch shorter and narrower than the parallel part. This lat ter is formed of two pieces of brass; one above the steel, and the other below it, to which they are securely rivetted. The stock and blade arc even with each other at the out side and left end; the projecting part of the steel, in the line of the pattern, lies upon the latter, preventing the square from tilting, and that at the end places the stock out of the way when the edge of the blade requires to be repaired. As it is in some eases necessary that the square should be used when placed upon the farther side of the work, the two parts or the stock should both be accurately at right angles to the blade ; the best way to secure which properly is this :—After having made the holes which are to receive the rivets, remove the steel, and pin the two branches of the stock together ; then, with the jEkne, make the inner edges straight and even with each other. In or der to prove that the blade is at right angles with the stock, take a broad scale of brass, having its edges truly straight and parallel, a thing by no means difficult to pro cure, and, first applying the square to one edge, draw a line across the scale with the dividing knife, and then, ap plying it to the other edge of the scale, with the same side uppermost, draw another line extremely near the former. If the parts are right, it is evident the two lines will be pa rallel to each other ; but if they are inclined, half their inclination is the error of the tool, which must be altered by grinding the edge of the steel blade. It is almost un necessary to observe, that the use of the square in parallel dividing, is precisely the same as that of the index in an gular dividing.