Mr Ramsdell followed the next in time; and perhaps gained greater credit in his line of business, than any pre ceding artist. The dividing-engine, which he invented, by rendering small instruments ahnost as accurate as large ones had been before, will shed lasting and well-merited honour upon his name. In his larger works, however, he was not so happy, although they were graduated better than any previous to his day. Too vain of invention, he despised the patterns left by his predecessors, and gave to his works complex and unsteady forms.
The late Mr John Trought on , about twelve years young er than Ramsden, was equal, if not superior, to him in the graduation of instruments. But his chief works, perform ed in his own attic, were destined to shed honour upon the names of the first mathematical instrument sellers in Lon don ; and by the time his merit became known to the pub lie, his younger brother had taken the lead. But we must close these short biographical sketches with the labours of the dead.
We are aware that these desultory remarks will ill sup ply the place of a regular history of the art ; a thing which we cannot attempt, and which is perhaps impossible. To describe things as they arc, and to give the methods as hitherto practised by eminent men, will certainly be more useful: and in doing this, as often as they have written upon the subject, their different modes of practice shall be given in their own words.
Of those numerous contrivances, which have neither in genuity of conception, nor have been found useful in their day, we shall say nothing ; for it would be idle to obstruct their natural passage to oblivion.
For the sake of perspicuity, we shall divide this article into three Sections, containing, I. Common graduation, 2. Engine graduation, and 3. Original graduation.
Ix this section of our article will be shewn the method of taking copies from a pattern, which has already been Laid down originally ; or, as indeed is now generally prac tised, taking copies from a copy. This part also includes original dividing, in cases where the usual patterns do not apply, and where the utmost degree of accuracy is not re quired.
It will here be necessary to describe the apparatus or tools used by the person who divides in common. And first the dividing plate must be noticed. Its dimension va
ries from 14 to 30 inches in diameter. It is either an en tire plate, or a broad rim, connected with the centre by four or more radii, and rendered inflexible by circular rings, or edge bars underneath. The extreme border is divided into degrees and quarters ; and just within this another circle, into degrees and third parts. Within are usually put such numbers as are required for the dial of the pe rambulator; Gunter's line of numbers arranged in a cir cle, and other logarithmic lines, are sometimes inserted. There are also often to he found tangents in hundredth parts of the radius, and the difference of the hypothenuse and base, as applied to the theodolite ; also the equation of time for dialling ; the points of the compass, Ste.
There is always in the centre of the plate a circular hole, which should be truly perpendicular to the surface. Into this is nicely fitted a pin or arbor, which also fits the cen tre hole in the circle or are to be divided; and while the operation is carried on, is the principal connection between them. Fig. 1. Plate CCLXXVIII. chews the dividing plate, and its connection with a compass ring : also a pair of holdfasts for keeping the work from turning round. One of these fastenings would be sufficient, were it not necessary to remove one of them, when its position ob structs the work. An index of tempered steel, usually made of a saw blade, has one of its edges made very straight: At one end a plate of brass, as at A, is rivetted fast to this index, having at its extremity a right angular notch, reaching a little beyond the blade : the angular point should he exactly in a line with tire edge of the index. This notch receiving the arbor of the plate, will always di rect its edge to the centr e. The length of the index should be equal to the radius of the plate. To the exterior end, and below it. is fixed a secondary index 13, reaching as far inwards as the original lines of the plate extend, which also must have its edge directed to the centre ; but it need not he exactly in the line of the other, and will he better seen if it is placed a little to the right. The figure slims this.
and also an arrangement of nuts and screws, by w hich the ,.