In dividing a common thermometer, several points, 12 or 15 degrees apart, are marked off, according with a standard one ; these, always unequal, are filled up with equal parts. The use of the dividers cannot be better exemplified than in this case : Say the distance from one mark to the next is 15°, the operator knows the value of his time better than to do this at two operations ; instead of •first dividing the space into three or five, he guesses or estimates the distance of 1°, and running the tool over the space almost as quick as he can count its steps, sees how much he has erred ; a second or third trial never fails to give him the proper distance. The clots in these trials, two of which should never be made in the same line, are barely to be seen by the glass, and he wants the last, that, by re peating the steps with a greater pressure, he may make the dots sufficiently large to receive the point of the dividing knife. It may be mentioned, that the operator does not draw a line in the direction of his work : without such help lie learns by practice to plant his points in the direct course.
Sector and plane scale patterns are divided from a diago nal scale, with the square and dividing-knife : the whole length of the scale is equal to the radius of the sector, and is divided into 1000 parts, and a lower subdivision is ob tained by estimation. The value of each division is picked up among the diagonals, according to tables of natural or logarithmic sines, tangents, Sec. Whoever wishes for full information upon this subject, may consult the Select lJochanical Exercises of the late celebrated James Ferguson; every table is there given, and not a figure more or less than what is required. The practical part of what is here referred to, Mr Ferguson learned from the fir st of the three Troughtons, to whom his youngest son James was appren tice, a youth of considerable promise, who died at the early age of 23 years.
If it should be thought that we have been unnecessarily diffuse in this department of the art, we would observe, that should any one, before he is fully acquainted with it, and habituated to the use of the attempt to practise the higher branches, he will most probably find himself un qualified for the task.
'hie. late ?Mr Henry Ilindley of York, about the year 1740, was the lirst who constructed an engine for gradu ating instruments, and which also served the purpose of cutting the teeth in clock wheels. N,Ve have it not in our power to give a particular account of this engine, but the late Mr Smeaton, in the Phil. Trans. for 1785, informs us that the plate was turned round by an endless screw, which having been cut with a tool that turned upon a centre at a distance equal to the radius of the plate, made it of smaller diameter in the middle, so that the screw throughout its whole length, acted in contact with the convex edge of the plate. Snicaton informs us, that both the screw and the
teeth in the plate, were produced from the original gradua tion of the plate. Mr Smeaton's paper here alluded to, is replete with general information upon the graduation of in struments ; but Hindley's method of original dividing. and his own improvements thereon, form the main subject. These will be briefly noticed in the next Section.
Ilindlcy, far removed from the metropolis, and perhaps knowing little how, in his time, the useful arts were culti vated there, was, by dint of his own native powers, making considerable progress in the improvement of his double profession of clock and instrument making. In the latter, however, lie must have wanted that constant employment which alone can ripen experience, and give full effect to execution. Ile died in the year 1771, at the age of 70 years.
An account of the next attempt to make a dividing en gine was published at Paris in 1768, by the Duke de Chaul nes ; every part of which is described with the utmost mi nuteness, and illustrated by fifteen folio plates, all full of figures. It will not, however, be to our purpose to give even an abstract of this ingenious work, on account of its having been superseded by better contrivances, a due at tention to which will occupy as many of our pages as can be appropriated to this subject. 'We may however ob serve, that the wheel of this engine is not turned round by an endless screw ; itself, together with the work to be gra duated, is acted on by a clamp and screw for slow motion ; by the latter, a division of the limb is brought to be bisect ed by the vertical wire of a fixed microscope, and then the corresponding division upon the work cut with a point and frame adapted to the purpose. We do not know that any small instruments were ever divided by the Duke de Chaul nes' engine, or that any large ones were clone according to the original method by which it was graduated. The me thod, highly interesting, and at that time altogether new, will find the notice it deserves in that Section of our present article to which it belongs.
It is, however, to the ingenuity of the late Mr Ramsden, that the world is indebted for engine dividing in its full ef fect. That artist, about the year 1766, produced an ,engine which, although it fell far short of his expectations, ex ceeded, in accuracy, the best dividing plate. It was fully competent to the division of common instruments for sur ...eying of land, Sce. but was deemed insufficient to produce that accuracy which is required for the purpose of finding the longitude at sea. This engine, about 30 inches in di ameter, after Ramsden had, about 1775, made another of nearly four feet, was sold to a nobleman in France for the purpose, of being lodged in his cabinet.