In most of the peper-making machines a partial vacuum is produced under the endless wire-web by means of large air-pumps. The atmos phere is thus made to press upon the pulp, and the moisture is forced through th; wire. Owing, however, to the change of stroke of the pistons, a uniform degree of vacuum cannot be maintained, so that the pulp receives various degrees of pressure, and consequently the paper is made of unequal thickness. To remedy this defect, by rendering the vacuum as uniform as possible, two patents were taken out in 1339 ; one to effect the purpose by substituting a revolving fan for the air-pump, and the other by using hydraulic air-pumps working on the principle of gas-holders, instead of the Common ones.
Various patents have been taken out for improvements in parts of the machinery, or for other machinery to be applied in various stages of the process. One was taken out by Mr. Dickinson for an apparatus to separate the knots and lumps from the pulp, by making the pulppa.ss through the periphery of a revolving cylinder constructed of an endless spiral wire attached to metal bars. Other improved processes for the same purpose have been invented. A patent was taken out by Messrs. Towgood and Smith, the object of which is to apply the size to the paper as it comes in a continuous sheet upon the endless wire-web. The operation in this apparatus is performed by rollers, the surfaces of which are supplied with size, which is transferred from them to the newly-made paper as it passes between the rollers. Mr. Dickinson has a patent for uniting two layers of pulp in order to produce paper of an extra thickness. The combination of two or more sheets of pulp has been long employed in the making of thick drawing-paper and Bristol-board by the process called couching ; but Mr. Dickinson's contrivance affords the means of doing it in a common paper-making machine. No attempt, however, can be made here to enumerate in detail the steps by which improvements have been introduced in the paper-machine; they are too numerous. Foreign inventors have done much in this matter. The names of Robert and Didot have already been mentioned; and we may also notice 31. Canon, who was the first to apply suction-pumps to the machines; and X Jequier, who devised a mode of making continuous paper with wire marks. Among English paper makers and machinists, the names of Brown, Crompton, Taylor, Barrett, Ibotson, Wilke, and Hollingworth may be mentioned as the introducers of valuable improvements in the continuous paper making. So wonderful is now the operation of the machine, that fine writing-paper can be made, sized with gelatine, dried, and cut into sheets, at the rate of 60 feet a minute in length, and 70 inches in width.
Varieties of Paper.—We shall now notice a few matters connected with the differences in the kinds of paper, without particular reference to the fact of their being hand-made or machine-made.
It may here be observed, that various wire-marks, or water-marks, as they are called, were formerly applied to paper to distinguish it. On the paper used by Caxton and the other early printers these marks consisted of an ox-head and star, a collared dog's head, a crown, a shield, a jug, &c. A head with a fool)] cap and bells gave name to the paper called foolscap ; and post paper seema to have derived its name from the mark of a horn, which was formerly carried by the postman, and blown to announce his arrival. Hand-made paper is now commonly marked with the name of the maker, and the date of the year when it was made.
The cutting of paper into eheeta of any desired size is an operation which has been greatly improved within the last few years. The best paper ia examined when made, to remove knots or specks, and to Lay aside damaged sheets. It is then counted into quires of 24 sheets, folded, and put up into reams of 20 quires. If the paper is for printing, the continuous web made at the machine is cut into sheets of a certain size, without any particular regard to the fineness of the cut ; but in the preparation of writing-paper, whether hand-made or machine-made, the cutting is very carefully attended to Hence there are cutting machines of two different classes, one to sever the web into sheets, and one to cut well-squared sheets of writing-paper. In the first of these kinds, the paper is generally coiled round a cylinder into a substantial mass, and this cylinder, whilo rotating, is brought. at intervals against a cutting edge in such a way as to cut through the paper. In the second kind much ingenuity has been shown ; and it is certain that the remarkable cheapening of writing-paper since the introduction of the penny post has been greatly due to the adoption of machines, instead of hand-cutting, for the severance of the paper into sheets. In Black's paper-cutting machine, a sharp knife fixed to a rack is made to traverse laterally by the motion of the rack. The paper is laid upon a bed, and is adjusted by a screw spindle. A back-plate is hinged at its lower edge, so as to be brought by pinching-screws to a proper position for the paper to be cut rectangularly ; and there ia a screw to raise and lower the knife. 3Iorgan's cutting-machine is intended to do the work with a small expenditure of power ; the cutting-knife rotates on an axis, and the paper advances horizontally to meet it, like the action of a circular saw in timber-cutting. In Ullmer'e machine there ia a contrivance for imparting to the cutting-knife a diagonal move ment, so as to enable it to make a draw-cut in an effective manner. There are many other paper-cutting machines, acting on similar prin ciples, but differing in details.