In ending the first part of this work, I gave my readers room to expect this part “within three months,” and am happy now to fulfil that engagement. Although these pages contain fewer errors than the former—an apology is due for those that have crept in: to which I add the promise that every thing shall be done to lessen them further in the future parts, and wholly to correct them before the work closes.

Page100,line2,for“:”,read ∷;
„126,„4,„“on its surface”read at its pitch line.
„126,„17,„“its height f g,”read the length required.
„129,„16,„“2,”read 4,
„„„20,„“imperfect,”read homely.
„144,„7,take away “alone.”
„„„8,for“usually”read chiefly.
„146,„23,for“the friction,”read it.
„147,„1,for“nothing,”read little or nothing.
In fig. 7 of Plate 19, slope the groove of both faces the same way.

A few words seem wanting to complete the description of the Cutting Engine above given. They relate principally to the cutter-frame and cutters. Although, with a view to celerity, I have shewn the cutter out of the frame ([fig. 4]) yet a common frame, carrying the arbor on points, may be used with propriety; and would often be an eligible substitute for the frame above described. In cutting bevel wheels however, either on this Machine or that [to be described], there is a form of the cutter frame which leaves less freedom of choice, as the cutter itself must have a peculiar form and position. To return to the cutter for spur wheels, their form (or section) depends on the degree of finish which the wheels require. For rough work they may be cylindrical on the face, the sides being under cut, so as to leave them thickest at the circumference—whence a certain coarseness of cut ensues, but without any injury to the spiral form. But, generally speaking, the cutters are best, when made a little tapering towards the edge, and toothed on both sides as well as on the circumference. The teeth should be tolerably fine, but not very so, unless great smoothness of surface were required: and we have seen above that, in this System, great smoothness is very seldom necessary, provided the obliquities be correct. I may add, that those cutters used on common engines, whose great rapidity compensates for the small number of their teeth, would not answer here, on account of the twisting motion in the wheel. But nothing prevents using cutters, so formed on the sides, as to round off the teeth in the act of cutting—only the cutter must be so thin as that its thickness, added to the aforesaid twist, may not make the spaces too wide. A little observation will render these things familiar to an attentive observer: nor shall this work conclude before all that I have gathered from long observation on this subject, be fully known to my readers.

J. W.

5, Bedford-street, Chorlton Row,
20th. November, 1822.

PART THIRD.

A NEW CENTURY OF