GAUGE, in the mechanical arts, is the name applied to a great variety of instruments, of which the object may be broadly stated to be the affording of in-creased facilities for comparing any two dimensions or distances. Wherever it is necessary for this to be done with a degree of accuracy unattainable by such means as the ordinary measuring rule affords, or for the same dimensions to be frequently measured with a maximum of speed and certainty, there will the hand-craftsman at once avail himself of some form of gauge. At the present day a due appreciation of the value of gauges is of growing importance to the mechanician, since they enable him greatly to improve the " fit" of the several portions of his machinery, whilst at the same time the labour expended in fitting is materially reduced. Indeed the system of making all similar parts " to gauge," so that in any number of machines they are interchangeable, is now effecting more than any other single cause for the improvement and cheapen-ing of mechanical substitutes for manual labour.

The gauges which come within the province of this article differ in two main particulars, according as they refer the measurements which can be made by them to some definite and established standard of length, or take cognizance only of an arbitrary or haphazard one. The obvious advantage of being able to record, and at any time again obtain with certainty, the thickness of a plate of metal, or any other gauged dimension, would have led one to sup-pose that for all except mere temporary purposes the gauges used would invariably be of the first kind—Standard Gauges, as we shall distinguish them. But the fact is un-happily far otherwise, at least as regards the important manufactures of sheetmetal and wire (which cannot be easily measured without some form of gauge), the result being that the thickness and diameters of these are expressed by vari-ous complicated and irregular series of numbers and letters, which have no reference either to each other or to any standard system of measurement. Of these arbitrary series the B.W.G. or Birmingham Wire Gauge may be taken as the type. The largest size of which it takes account is known as No. 0000, after which come 000, 00, 0, and then the numerals from 1 to 36, which last is the smallest size. It is frequently used for gauging the thick-ness of sheet metal as well as for wire, in spite of the ex-istence of the Birmingham Plate Gauge, which has an equally arbitrary series of its own, consisting of the same numbers (from 1 to 36) used in the reverse manner, the low numbers being the small sizes. Other arbitrary wire gauges also tend to add to the general confusion, amongst which may be mentioned the Lancashire Gauge, which takes an alphabet and a half, in addition to the numerals up to 80, for expressing the sizes of steel wire which are referred to it, but which nevertheless does not apply to " music wire," or " needle-wire," or sundry other special kinds of wire, which are favoured with separate gauges of their own. Of late years careful comparisons have more than once been independently made with a view to ascertaining the standard value of these incongruous systems, but the discrepancies iu the results only prove what might have been predicted, viz., that errors have crept in, and that those which profess to be alike differ amongst themselves, whilst there exists no satisfactory means of rectifying these errors. Their gradual and entire abolition therefore seems to be the only chance of real improvement, and it is earnestly to be hoped that the Standard Gauge originally suggested by Sir J. Whitworth, which is now largely employed, may soon entirely supersede them. In this system the sizes are directly referred to the English imperial standard of length, each being expressed by the number of thousandth parts of an inch which it contains. Thus No. 36 wire means wire -036 of an inch in diameter. Under the old systems this might have been either No. 20, No. 62, No. 3, or No. 18.

Examples of some of the usual forms of gauges are given below. For wire the simplest gauge consists of a steel plate with a series of holes drilled through it, each hole being numbered according to the series to which the gauge re-fers. By means of the Notched Gauge (fig. 1) sheet metal can be gauged by a similar mode of obtaining a more or less accurate fit. Bough gauges on the same principle are constantly employed also in workshop practice for comparing together internal or external diameters, &c.; and they serve the purpose well enough so long as the object is a mere comparison, without taking account of the amount of any minute difference which may exist. When a measure-ment of such differences is required, or direct reference to a standard system, recourse must be had to some form of gauge provided with means for enlarging them sufficiently to be readily recognizable. Sliding or Calliper Gauges, such as fig. 3, fulfil this requirement by having the graduated scale affixed to one of their arms and a vernier in connexion with the other. A V-gauge, which, instead of a series of notches round its edge, has only one long tapering notch, by the graduations of which the diameter of any wire that will enter it can be read off, is simple and tolerably efficient. So also is the kindred arrangement (fig, 5), in which a wire or plate can be inserted between a fixed pin and the edge of a revolving cam with graduated face. But perhaps on the whole the best and handiest form is the Micrometer Gauge (fig. 2), which, by means of a micrometer screw with a divided head, measures to the one-thousandth part of an inch, and in careful hands can render visible even smaller fractions. Gauges consisting of two arms jointed together like pincers are also used in certain trades, minute differences in the width of the jaws being magnified and rendered visible on a graduated arc at the opposite ends of the arms.

For special purposes gauges of many other forms are employed, some of which are of much greater delicacy, but these cannot be described here. The only others which remain to be mentioned are those of which the Plug and Collar Gauges (fig. 4) are the type, sets of which are now to be found in almost all mechanical workshops where the value of standard dimensions is recognized. Each gives only the one external or internal dimension for which it is made, but it gives that with the highest attainable accuracy, so that by carefully preserving a comparatively small num- ber of these for reference, and using them in conjunction with measuring machines, the most minute differences can be measured and noted in terms of the standard, so that exact sizes can at any future time be again obtained with- out appreciable error. (o. p. B. S.)