The mould used by paper-makers is a kind of sieve of an oblong shape, bottomed with the very finest wire strands, all of which run horizontally from end to end. From top to bottom, and about an inch apart, are placed "chain wires," and on the right-hand side of the mould the wire water-mark, which, together with the wire-marks, appears semi-transparent. The reason of this is that both water-mark and wires are slightly raised, and of course the pulp is thinner there than anywhere else. Any ordinary sheet of paper held up to the light will show this, and serve to extra illustrate the following diagram. [Illustration: PAPER-MAKER'S MOULD: JUG WATER-MARK.] Here CDEF is the mould which the workman drops into a vat of pulp, the fine strands run from G to H all the way down the mould, AA, &c., are the chain wires, and B is the water-mark, in this case a jug. The water in the pulp of course runs through the sieve, leaving a layer of soft matter, which after a while hardens into a sheet of paper. The water-mark was at one time the trade mark of the maker, but subsequently became merely a symbol denoting the size of the sheet of paper before it was folded. The smallest sheet was water-marked with a jug, as above, and termed "pot"; the next had a cap and bells, hence our term "foolscap"; the next a horn, hence "post". Others had a "crown," and so on. At the present day all water-marks have once more become trade symbols, and cannot be depended upon to afford any evidence of size; but at one time--_i.e._, before the year 1750--this was not so, and, therefore, these water-marks, irrespective of their antiquarian value, serve a useful purpose--namely, to point out in cases of doubt whether any given book is an octavo, quarto, or folio, or a variation of any of these sizes. To refer once more to the diagram. Take a sheet of paper supposed to have come from the mould and double it in half at the line AX. The water-mark will in that event appear in the centre of the half sheet, and the folded paper is of folio size. Now fold the paper the contrary way, and the water-mark will appear at the bottom, but cut in half; the paper thus folded is quarto (4to). Now fold it the contrary way again, and a section of the water-mark will appear at the top; the paper thus folded is octavo (8vo). We can go on folding, and in every subsequent case the watermark will appear at the edges, while, as the paper gets smaller and smaller, the sizes are styled 12mo, 16mo, 32mo, and so forth. In the example given, a book made of the sheet of paper in question would be a pot folio, pot 4to, pot 8vo, and so on; but as larger-sized papers were used, another book might be a post 8vo, or a crown 4to, &c., according to circumstances.