About one-third of these sizes have been constantly used, up to the present time, both by II. & Co., and by other persons to whom copies of these screw tackles have been supplied, and consequently many thousands of screws of these kinds have been made: this implies the continual necessity for repairs and alterations in old works, which can only be accomplished by retaining the original sizes.

Since the period at which II. & Co. made their screw lathe, they have employed the aliquot threads for all screws above half an inch; indeed, most of these have also been cut in the screw lathe. To have introduced the same method in the small binding screws which are not made in the screw lathe, but with the die-stocks and chasing tools, would have doubled the number of their working-screw tackle, and the attendant apparatus; with the risk of confusion from the increased number, but without commensurate advantage as regards the purposes to which they are applied.

Doubtless the same reasons have operated in numerous other factories, as the long existence of good useful tools has often lessened, if not annulled, the advantage to be derived from a change which refers more immediately to engineering works; and in which a partial remedy is supplied, as steam-engines, fee. are frequently accompanied with spare bolts and nuts, and also with corresponding screw apparatus, to be employed in repairs; the additional cost of such parts being insignificant, compared with the value of the machinery itself.

Thirdly: Unless the standard sizes of screws become inconveniently numerous, many useful kinds must be omitted, or treated as exceptions. For instance, in ordinary binding screws, more particularly in the smaller sizes, two if not three degrees of coarseness should exist for every diameter, and which might be denominated the coarse, medium, and fine series; and again, particular circumstances require that threads should be of shallow or of deep angular sections, or that the threads should be rounded, square, or of some other kinds; in this way alone, a fitness for all conditions would inconveniently augment the number of the standards.

In many cases besides, screws of several diameters are made of the one pitch. In order, for example, that the hole when worn may be tapped afresh, and fitted with screws of the same pitch or thread, but a trifle larger; * or that a partially worn screw may be corrected with the dies or in the lathe, and fitted with a smaller nut of the same pitch. A succession of tap3 of the same pitch also readily permits a larger screw to be employed, when that of smaller diameter has been found to break, either from an error of judgment in the first construction of the machine, or from its being accidentally submitted to a strain greater than it was intended ever to bear.†

• This is done in some of the patent screws for joinery work, so that when the thread in the wood is deteriorated from the frequent removal of the screw, another of the same pitch, hut larger diameter, may be substituted.

† Mr. Clerneut has screw taps of 3/4, 7/8, 1, 1 1/8, 1 1/4, 1 3/8, 1 1/2, etc, inch diameter, and all of seven threads per inch. Holtzapfiel and Co. have taps, etc, for screws of ten threads per inch of fifteen different kinds, which are used for slides and adjustments, besides less extensive repetitions of other threads.

It is also in some cases requisite to have right and left hand screws of the same. pitch, that, amongst other purposes, they may effect simultaneous yet opposite adjustments in machinery as in some universal chucks: and also some few screws, the threads of which arc double, triple, quadruple, and so forth, giving to screws of small diameters considerable rapidity of pitch or traverse, or a fixed ratio to other screws associated with them, in the same piece of mechanism.

Fourthly: Friction prevents the strict maintenance of standard gages for screws. The universality of system, to be perfect, should admit that a bolt made this year in London, should agree with a nut made ten or fifty years hence in Manchester, which is not called for, nor perhaps possible, if an absolute fit be required in reference to this we must commence by a small digression.

In comparing the Exchequer Standard Yard Measure with the copies made from it, friction in no way interferes, as the two measures are successively observed through two fixed microscopes, as before adverted to. But we cannot thus measure a cylinder, as either callipers, or a counterpart cylinder placet' contact, must be employed as the test; and each time of trial the cylinder is absolutely, although very slightly worn, by the traverse of the surfaces against each other; the form of the cylidrical gage being simple, to increase its durability, it is worked to the figure after having been hardened.

In measuring a screw, the callipers are insufficient, and the one screw must be screwed into the other: from this trial much more motion, friction, and abrasion arise. Further, the screw gage cannot, from its complex form, be readily figured after the material has been hardened; and if hardened subsequently to the helical form having been given, the measure becomes, in some degree, altered, from the action of the fire and water, which is a fatal objection.

Under ordinary and proper management, the production of a number of similar pieces may be obtained with sufficient exactitude, by giving to the tool some constant condition. For example, a hundred nuts tapped with the same tap, will be very nearly alike in their thread; and a hundred screws passed through the hole of a screw-plate, will similarly agree in size, because of the nearly constant dimensions of the tools, for a moderate period.

In practice, the same relative constancy is given to the dies of die-stocks and bolt-screwing engines, and partly so to the tools of the screw-cutting lathe. Sometimes the pressure or adjusting screw has graduations or a micrometer; and numerous contrivances of eccentrics, cams, and stops, are employed to effect the purpose of bringing the die or turning-tool to one constant position, for each succeeding screw; these matters are too varied and general to require more minute notice. Part of such modes may serve sufficiently well for ten, or a hundred screws, provided that no accident occur to the tool; but if it were attempted to extend this mode to a thousand, or a hundred thousand pieces, the same tool could not, even without accident, endure the trial: it would have become not only unfit for cutting, but also so far worn away as to leave the last of the works materially larger than the first.