Note BM, referring both to the Table on Rectilinear Saws, page 699, and to the

Table on Circular Sawn, page 784.

On the Gages at present used for measuring the thicknesses of sheet metals and wires, and proposals for a new system of Gages, founded on the decimal subdivision of the Standard Inch.

In setting out the Tables of the Dimensions of Saws, the author could only express their several thicknesses, in the measure always employed for that purpose, namely, in the sizes or numbers of the "Birmingham wire gage," and to render these measures intelligible to the general reader, the author then determined to introduce in this Appendix - first, the exact values of the principal gages in use for sheet metals and wires, a subject he believes to have been hitherto overlooked; and secondly, a proposal he has long desired to see carried out, namely, an easy and exact system of gages for sheet metals, wires, and general purposes, founded on the decimal division of the inch; and in which system the nomenclature should be so completely associated with the actual measures, as to convey to the mind, even in the absence of the gages themselves, a very close idea of the several spaces of the gage, or of the thicknesses or sizes of the works measured thereby.

It is to be observed at the outset, that the gages for measuring wires and sheet metals, are usually thick plates of steel of several sizes and forms, around and near the edges of which are first drilled various holes, the next step is to saw a notch from the edge into every hole, saws of the widths of the several notches being used; and lastly, little parallel plates of steel, called drifts, which are hardened and tempered, are driven into the notches, in order to smooth the sides of the same and render them of uniform width, after the manner of various other applications of drifts, explained at pages 883 to 885.

It should be further observed that the Birmingham and other gages seem to have been originated in great measure accidentally, or almost by the eye alone, and without any attempt at system, either as regards the values of the intervals between the successive measures or numbers, or their correspondence with the subdivisions of the inch. And as moreover gages, nominally the same, have been made by various manufacturers with insufficient aim at unity of measures, some irregularity thence exists amongst the gages in common use, notwithstanding that they may be nominally alike.

In ascertaining the precise measures of the principal gages, the author has had the valuable co-operation of Messrs. Stubs, of Warrington, who manufacture a large number of these gages, and who tested the drifts they employ, by means of a sliding gage constructed by Holtzapffel & Co., for reading off quantities to the thousandth part of an inch, by means of a vernier; the results of these admeasurements are stated in the three sections of the accompanying table.

The three series of measures or gages particularised in the annexed table, have no relation whatever to one another; for example, the numbers 10 of the table are respectively different and undefined quantities, or are neither aliquot nor direct fractional parts of the inch, as the number 10 notches, are severally .134, .024, and •190 of an inch wide; and other similar numbers are also unrelated.

The approximate measures of any one of these three series may, perhaps, be moderately familiar to those artizans who use that particular gage, but these same artizans will probably be as little informed of the two other gages, as the generality of individuals to whom the whole of those, and other arbitrary ill-defined measures are vague and confused; because their nomenclatures have no relation whatever, either to one another, or to our general standard of such quantities, namely, ordinary linear measure; or, in other words, the standard foot and inch.

The following explanatory remarks on the three gages specified in the table, and certain other gages derived from them, will show the complicated and uncertain nature of the subject of measures, for wires, sheet metals, and various small works.