GRADUATING INSTRUMENTS. The name of graduation is given to the marking of degrees and fractions of a degree, on mathe matical and astronomical instruments. This graduation, or, as the workmen more generally style it, dividing, is performed in two ways, by making a copy of a system of divisions already existing, or by original dividing. The straight scales and rules which are in common use in cases of drawing instruments, are divided thus :—The original pattern, and the scale on which the copy is to be laid, are placed side by side ; a straight edge, with a shoulder at right angles, like a carpenter's square, is made to slide along the original, stopping at each division, when a corresponding stroke is cut, by the dividing knife on the copy. With care and practice, this method admits of consider able accuracy. By making the straight edge turn on the centre of a divided circle, the divisions of that circle may be copied upon any concentric circle. Common protractors are thus divided, and scales upon circular limbs. The original circle, which may have several orders of divisions for different pur poses, is called a dividing plate.
Theodolites and ordinary circular instru ments were thus divided, previous to the in vention by Ramsden of his dividing engine. The general principle of this engine may per haps be understood by the following descrip tion :—A horizontal circle of four feet diameter turns upon a vertical axis ; the outer edge is retched, or notched, by an endless screw, one revolution of which carries the circle round 10, or one-sixth of a degree ; the pressure of the foot upon a treadle turns the screw for ward, and there is a series of very ingenious contrivances which enable the divider to turn the screw through any portion of its revolution at each descent of the treadle, and which restores the position of the parts, when the foot is taken off, without allowing any return motion to the screw. The circle to be
is fixed upon the dividing engine, and made concentric with it and a division cut after each pressure of the foot. The Board of Longitude gave Ramsden a reward of 3001. for the in vention of this machine, and 3151. for the machine itself, leaving it, during pleasure, in his possession, on condition that he would divide sextants at six shillings and octants at three, for other mathematical instrument makers. Machines of a similar kind, with some alterations and improvements have since been constructed by John Troughton, Edward Tronghton, and others, and these are still employed in all instruments which are not large enough, or not sufficiently valuable, to require original dividing.
Dividing engines have been constructed somewhat differently by Reichenbach and others in Germany, and by Gambey in Paris. Much of the German division is excellent, and probably superior to any English engine dividing. It is understood to be performed by copying. A large circle having been divided originally with great care, the copy is placed upon it, and concentric with it. A microscope is fixed independently over the divided circle, the divisions are brought in succession under the wires of the micrometer, and a line is out in the copy after each bisection. This pro cess is much more tedious than the English engine dividing, but it admits of the greatest accuracy when the workman is careful and expert.
Original dividing, by which the dividing engines themselves are graduated, and which is employed for the highest class of astrono mical instruments, is a process requiring all the resources of delicate mechanical and mathematical art.