The celebrated Mr Watt's. Watt, of Birmingham, in 1780, obtained a patent for a method of copying recent writings. The substance of his process is as follows: A sheet of thin unsized paper is wetted, and then laid between two woollen cloths, to take away the superabundant moisture. This paper is applied upon the surface of the fresh written letter, and the letter and wet paper, thus in contact, are passed through a rolling-press, or subjected to the action of a screw-press. The portable printing-presses made by Ruthven likewise answer well for this purpose. After the operation of the press, the thin paper is found to have received an impression of the letter reversed; but this impression is legible in the right direction when it is looked at through the transparent substance of the paper. The liquid used to wet the thin paper may be water, or a liquid composed, as Mr Watt directs, of vinegar, boracic acid, oyster shells, and gall-nuts. In this composition the gall-nuts seem to be the essential ingredient, their effect being to render the impression blacker, by combining with the superabundant iron which may exist in the ink wherewith the letter was written. The letters are to be written with writing ink, made in the usual way, of a decoction of gall-nuts, sulphate of iron, and gum Arabic.

Repertory of Arts, Vol. I. p. 13.

Mr W. Bell's patent was granted for a method of Bell's copying letters in a letter-book. For this purpose, a letter-book is made of thin unsized paper, and the leaf of the book on which the copy is to be taken is wetted; the letter freshly written is applied to the wetted leaf; the book is then shut, and subjected to the action of a screw-press. The impression of the letter is left on the leaf, and is read by looking at the other side of the leaf. The advantage of this method is, that copies of letters in a letter-book form a regular series, and are received as evidence in courts of law, where copies on a detached sheet would be objected to.

Another mode of producing duplicates of writings is that for which Ralph Wedgwood, of Piccadilly, obtained a patent in 1806. He employs, 1st, A sheet of paper, over both sides of which printer's ink is spread; this is allowed to dry during six weeks, between leaves of blotting paper; 2dly, A smooth pewter or copper plate. 3dly, On the metal plate is laid a leaf of letter paper; over it the blackened paper before mentioned; and over this a leaf of thin paper, previously oiled, that it may be the more transparent. 4thly, On the paper thus disposed, the writing is performed by a style of agate, ground and polished to a smooth round point. The effect is, that the letter paper receives an impression from the blackened paper, and this impression is in the right direction, and constitutes the letter to be used as the original. The upper oiled paper receives an impression which is inverted, but may be read in the right direction, by looking through the paper. This constitutes the duplicate or copy. (Repertory of Arts, Vol. XXVII. 1807.)

A third class of copying machines is composed of those in which the hand of one writer gives motion to two pens at the same time, so as to produce two similar writings. Hawkins's polygraph, for which a patent was granted, is of this nature. It consists of two pens placed in a frame, and connected by joints, so that when any motion is given to the one pen, the second pen shall perform a similar motion. Whilst a person writes a letter with one of these pens, the other pen forms a copy or duplicate of the letter on a sheet of paper, to which this second pen is applied.

The apparatus packs in the form of a portable writing-desk.

M. Brunel obtained a patent for an instrument of a similar kind in 1799. (Repertory of Arts, Vol. XIII. 1800.)

Ralph Wedgwood's apparatus, for which he obtained a patent in 1808, consists of a certain disposition of two leaves of paper, by folding or rolling. The part of the sheet on which a line of the original is written, is brought close to the part of the other sheet on which the corresponding line of the duplicate is written. The line of the original and of the duplicate are formed at the same time, by two pens fixed in the socket of one handle. The handle is held like a pen in the usual way. (Repertory of Arts, Vol. XXXI. 1809.)

Franklin proposed a mode of copying letters, which consisted in writing the letter with gummed ink; this was sanded over with emery in powder. The letter, thus prepared, was laid upon a smooth plate of pewter, and passed through an engraver's rolling-press. The impression of the emery was left on the pewter, and printing-ink being applied to the pewter plate, an impression was to be taken, which was to serve as a copy of the letter.

Inscriptions cut in marble or other stone, are copied by laying a sheet of white paper on the inscription, and by rubbing once over the surface of the inscribed paper a bunch of rags, dipped in pulverized black lead. If the inscription is cut on the stone, the letters on the paper appear white on a dark ground. If the letters of the inscription project above the plain surface of the stone, the letters on the paper are dark, and the ground is white.