GRADUATION is the name given to the most delicate, dif ficult, and important branch of mathematical instrument making : it gives to the instrument the means of ascer taining the dimensions of objects, or their distance Irom each other, according to its nature, whether linear or an gular measure is required.
The substance of this article was intended to have been placed under the equally appropriate title of DIVIDING, and was partly written with that intention. Our having indiscriminately used both these terms, is owing not only to this circumstance, but also because the latter is exclu sively used by the workman.
We believe that in every country in Europe, clo6k making was the earlier art, and that clock-makers were the first who fabricated mathematical instruments. But as the excellence of the time-piece depends not at all upon the accuracy of the division of its dial-plate, we may suppose that instruments, the perfection of which rests principally upon the correctness of division, came from their hands in a very rude state.
As clocks, however, must at first have raised clock makers out of brasiers, smiths, or other workers in me tal nearest allied to the nature of the work; so instru ments must have made instrument-makers, and for this purpose the clock-maker was more than half formed. One would think, indeed, that makers of compasses, dials, rules, astrolabes, &c. from the great usefulness of these instruments, must have existed prior to clock-makers, and of course the graduation of them; but however that might be, if there was about the middle of the 17th cen tury any such distinct trade in this country, those who practised it were little thought of by men of science; for the instruments invented by Hook were made by Tompion; and both Tompion and Graham in succession made instru ments for the royal observatory.
It was the opinion of the late Mr Smeaton, that Mr Abram Sharp, the assistant of Flamstead, was the first AVM cut accurate divisions upon astronomical instruments ; he having, about the year 1639, constructed and graduated for the royal observatory, a mural sextant of 61 feet radius, which in the hands of Flamstead rendered essential ser vice to astronomy. Whether Mr Sharp was bred up to
any mechanical business, or whether the whole was the effort of his own genius, is now unknown.
There were, however, instrument-makers in the early part of the 18th century, who, in the art of dividing, might at least have equalled those celebrated clock and watch makers, whose names have been mentioned. Some of the works of Rowley are still extant, and bear such evident proofs of neatness and accuracy, that many a workman of the present day might be proud to own them. The elder Sis son, contemporary with, or a little later than Rowley, like him, constructed and graduated large instruments with success ; but neither the manner of performing the work of graduation, when beyond tile limits of their dividing plates, nor the method pursued by Sharp, has been re corded. It was in the workshop of Sisson, that the eight feet mural quadrant, several large zenith sectors, &c. of Graham, were executed. That the latter should be em ployed in the construction of them, rather than any one who was exclusively an instrument-maker, was owing, no doubt, to his superior abilities as a general mechanician, his knowledge in astronomy, and his proficiency in making observations, as well as to his sound judgment, and nice execution of the most essential parts of whatever he un dertook to construct.
About the year 1727, the late Mr Bird, then a rustic lad of Bishop Aukland, observing the unequal divisions and coarse-engraving of a clock dial-plate, upon doing one himself. The success of this attempt was the first step that led to the developement of powers, which, during a long- life, proved beneficial to science, and ren dered his name an honour to his country. It was from the elder Sisson, to whom he served a short apprenticeship, and his acquaintance with Graham, that Bird learned every thing that was not derived from his own resources. Bird and the younger Sisson were contemporaries and rivals for fame, were both men of considerable abilities and appli cation. But the superior ingenuity of the latter lost its effect with the best informed, when brought into compe tition with the accurate execution and sound judgment of the former.