The methods of original graduation above described are not practically adopted except for the largest and most important astronomical or geodetical instruments. Ordinary instruments are graduated by dividing plates or engines which copy and adapt a set of already existing divisions. The dividing-plate, which is used for common pur poses, such as dividing compass rings, etc., is a divided circle with a steel straight-edge, made movable on the axis or arbor of the plate in such a manner that its edge during every part of its revolution shall fall in the exact line from center to circumference. The ring, protractor, or other instrument to be divided, is clamped upon the plate with its center exactly coinciding with that of the plate, and the straight-edge is moved round, and made to halt at the required divisions on the circumference of the dividing plate, and by using the steel straight-edge as a guide, corresponding divisions are marked of upon the concentric arc of the instrument to be divided. The dividing-engine is a very complex machine, requiring the greatest accuracy and care in its construction; so much so, that the possession of a good one affords the means of obtaining a very good income, with a moderate amount of labor in using it. Such was the case with the
instrument of Mr. Parsons of London, who for many years divided a large proportion of the best theodolites, sextants, etc., that were made in this country. Among the most celebrated dividing-engines may be mentioned those of Ramsden, Troughton, Simms, and Ross. A detailed account of the construction of these would far exceed our limits. Their principal parts consist of a large circle divided with extreme care by original graduation. This wheel is racked on its edge with teeth as equal and accurate as the divisions; a very carefully constructed endless screw works in these teeth, and is moved through any given number of revolutions, or any measured fraction of a revolution, by means of a treadle or other suitable power, thus making the requisite steps for each division; another part of the machine cutting a fine line at the moment of the halt of each step.
These divisions are cut upon an arc of silver, gold, or platinum, which is soldered or, inlaid upon the limb of the instrument, the precious metals being used, on account of the oxidation to which common metals are liable.
•