In the patent dicstocks the cutting is so much facilitated, that the labour is reduced perhaps to less than the half of that required with the old-fashioned and nearly semicircular dies, fig. 678; hut when the guidance is too far sacrificed, the greedy action of the diet is a source of mischief. For instance, the instrument, fig. 589, with three dies moving simultaneously, has been superseded because of its risk of cutting irregular or "drunken "screws: for if, from the dies being improperly placed, the thread does not exactly meet, or lead into itself in the first revolution of the dies, hut finds its way in with a break in the curve, this break continues unto the end; as the three points of bearing, so to speak, being narrow, they may pursue the irregular line, thus giving to the dicstock a rolling or "wabbling" motion, instead of a steady quiet descent. This fault is also liable to occur in every diestock, in which there is any risk of the blank cylinder not being placed truly axial, from the dies touching only by points or narrow edges, instead of against a fair proportion of the curve; but, when the dies are moderately broad, there is more chance of the defect being afterwards corrected.

Subsequently to the introduction, by Messrs. Whitworth, of their screw-stock, shown in fig. 589, they invented a diestock with four dies, the one side of each of which was radial. The dies acted two at a time, just like turning tools, they were quite free from rubbing, and were simultaneously advanced by two wedges yoked together by a cross piece, and moved by one screw. This ingenious plan was not however regularly adopted, on account of the deficiency of the guiding power, as the screw was supported between four series of points; but it gave rise to the mode explained in fig. 590, in which the broad guide is judiciously introduced.

It is difficult, however, to decide fairly and impartially upon the respective merits of diestocks, many of which approach very nearly to one another; as whether the facility of cutting, or the truth of the screw, or any other point be made the standard of comparison, it is a judgment which must necessarily be given rather by opinion than by measure; and the conditions which arc aimed at in all screw-stocks, are in strictness unattainable in any, owing to the varying dimensions of the object to be produced.

From many reasons, it appears needless to strain the application of the diestock to the production of long screws, which require either a very precise total length, or a very precise equa-lity in their several parts. The main inconvenience results from unavoidably mixing the guiding and cutting in the same part of the one instrument; an instrument which acts by producing a series of copies of the few threads in the dies, and which copies become collectively the long screw. This mode of proceeding is equally as impolitic, as setting out a line of 50 or 100 inches long, with a little rule measuring only one or two inches.

Neither can it be desirable to cut long,and consequently slender screws, by an instrument used as a double ended lever, in the application of which, the screw, supported generally at the one end in the vice, is very liable to be bent; as any small disturbing force at the end of the stock, is multiplied in the same proportion as the difference between the radii of the work and instrument. The liability to bend the screw is reduced to the minimum, in Mr. Allan's simple apparatus, (p. 582,) for cutting the screws for dividing engines and other superior works, but which mode is not adapted to ordinary screws; the machines for screwing bolts entail also little risk of bending the screw.

On the whole it appears questionable whether for short screws, which are the legitimate works of the diestock, some of the better forms of the two part dies are not as good as any; and on the other hand it appears quite certain that for those screws in which particular accuracy is of real importance, that the screw cutting engine or turning lathe is beyond comparison more proper. This valuable engine will be soon referred to, and in it the distinct processes of guiding and of cutting are completely detached, and each may independently receive the most favourable conditions; whereas in all the modifications of the screw-stock they are more or less intimately commingled, and are to a certain degree antagonists.

The screw-cutting lathe has also the advantage that one good screw having been obtained as a guide, its relative degree of perfection is directly imparted to the work, and it may be employed for cutting very coarse or very fine screws, or in fact any of the various kinds referred to in the preliminary description.*

• Some remarks will be offered in the last section, on the proportions and forms of screws of a variety of kinds.