* The apparatus was fitted to the second screw-lathe of those described, and the inclined bar was placed on temporary woodon standards, † Subsequently Sir J. Barton, Comptroller of the Mint, etc.

646 barton's application of two pairs of dies.

Mr. Barton employed two pairs of dies upon the one screw; the dies were fixed at various distances asunder upon one frame or bar, and the screw was passed through them. This was found to distribute the minute errors so completely, that little remained to be desired; as it is obvious that at those parts where the screw was too coarse, the outer sides of the threads were cut, and which tended to shorten the screw; and where it was too short, the inner sides were cut, which tended to lengthen the screw; in fact the two parts temporarily situated within the dies, were continually endeavouring to approximate themselves to the fixed unvarying distance, at which the dies were for the time placed.†

Mr. Maudslay did not restrict his attention to the correction of the screw for the purposes of science, but he also effected a great many improvements in the system of taps and dies, by which they were made to cut instead of squeeze: as to him are due the introduction of the three cutting edges, and the division of the taps into the series of three, namely, the entering or taper tap, the middle, and the plug tap, by which shallow holes maudslay's improvement in screwing tools, etc. 647 or dead holes, in cast iron, can be safely tapped with full threads, a matter before impossible.

* The microscope had been long used in the process of graduating instruments, but this invaluable mode of employing two microscopes in combination, was first successfully practised by Mr. Troughton.

+ Mr. Barton informed the author that he employed the screw corrected in the above manner, in his engraving machine employed for cutting with the diamond, the lines as fine as 2000 in the inch, on the steel dies referred to in the note on page 42, vol. i.; and he said " that such was the accuracy of the mechanism, that if a line were missed, the machine could be set back for its insertion without any difference being perceptible." The author unintentionally ascribed the first application of the diamond to turning steel, to Sir John Barton (see note, page 179, vol. i.), whereas it had been used long before by Ramsden in cutting the hardened-steel screw for his rectilinear dividing engine. See his tract, pages 14,15.

X The accuracy of a screw cut by Mr. Maudslay, and employed in Mr. Donkin's rectilinear dividing engine, is indisputably shown at page 654 of this volume.

This engineer also made a scries of taps, from six inches diameter, for attaching the pistons of steam-engines to their piston rods, to the smallest used in screw-plates for watchwork. The diameters of these taps were derived from the ordinary subdivision of the inch into eighths and sixteenths; and their threads were jointly determined by the respective strength of each screw, and the choice of defined rates, such as 3, 3 1/4, 4, 4 1/2, 6, 8, etc., threads per inch. To have employed one constant angle or proportion between the diameter and pitch, would have introduced many fractions into the rates of pitch, and an irregularity of strength in the screws themselves. The formation of these taps was rendered comparatively easy, after he had introduced the true original screw and the system of change wheels, as a common practical apparatus; many copies of these screw threads have found their way to other workshops, and have served to influence the construction of similar tools of various proportions.

Indeed, I believe it may be fairly advanced, that during the period from 1800 to 1810, Mr. Maudslay effected nearly the entire change from the old,imperfect, and accidental practice of screw-making, referred to at page 635, to the modern, exact and systematic mode now generally followed by engineers; and he pursued the subject of the screw with more or less ardour, and at an enormous expense, until his death in 1S35. The results have been so important, and arc so well appreciated amongst mechanical men generally, that they may be considered fully to deserve the short digression to which they have led.

In 1816, Mr. Allan was rewarded by the Society of Arts for method of cutting micrometer screws with dies: the representation and description of the instrument will be found at page

582, where it is shown in the act of cutting an original screw with an inclined knife. Micrometer screws are cut in this apparatus much in the same manner, except that about one-third of the thread is cut with the large die, fig. 535, the inner curvature of which agrees with the curvature of the blank cylinder, and the screw is finished with the smaller die, 536, cut by an original of the same diameter as the finished screw. The piece prepared for the screw must always have two cylindrical ends to fit the semicircular bearings b b; this arrangement prevents the screw from being bent in the process of cutting, but which latter operation is accomplished entirely with the dies.*

648 clement's mode of originating screws.

About the year 1820, Mr. Clement devised and put in practice a peculiar mode for originating the guide-screw of his screw-lathe, the steps of which plan will be now described.

1. He procured from Scotland some hand-screw tools cut over a hob with concentric grooves; and to prevent the ridges or points of the screw tools, from being cut square across the end, the rest was inclined to compensate for the want of angle in the hob or cutter.

2. A brass screw was struck by hand, or chased with the tool 1.

3. The screw 2, was fixed at the back of a traversing mandrel, and clipped between two pieces of wood or dies to serve as a guide, whilst

4. A more perfect guide-screw was cut with a fixed tool, and substituted on the mandrel for 3: as Mr. Clement considered the movement derived from the opposite sides of the one screw, became the mean of the two sides, and corrected any irregularities of angle,- or of drunkenness.