_Tub-sizing._—The foregoing description is of a machine for the manufacture of engine-sized papers; some slight modifications are necessary in the case of papers that are tub-sized on the machine. In making the cheaper qualities of tub-sized papers, the paper, after being partially dried over a few cylinders, is passed through a vessel containing a {161} solution of gelatine (see p. 162). It then goes between a pair of rollers, which press out the excess of size, and then again over drying-cylinders. The vessel or trough containing the size is filled to overflowing from a tank placed at a higher level; the overflow passes into a lower vessel, from which it is pumped up to the top tank. The size is kept warm by means of a coil through which steam passes placed in one or other of the vessels containing it. The paper passes between a pair of rollers placed in a trough and dipping into the size. The size may be made to flow through a pipe pierced with a number of holes, the stream running directly on to the web of paper. In the other, and perhaps most general way, at least for the better qualities, the paper is wound off immediately after leaving the last drying-cylinder A (Fig. 55), and sized at some future time, or it may be passed directly into the vat B containing the size. After passing between the squeezing-rolls. C, it is generally wound off as at D; and after having stood some time {162} to allow the size to be evenly absorbed by the paper, it is wound off from F, and passed over the cylinders H, of which there may be a very great number, some machines having over 300 of them. These cylinders are made of light spars of wood; inside them, and revolving rapidly in an opposite direction, are the fans G. The paper, in travelling over these drums, is only slowly dried, and is supposed by this means to be more perfectly sized, and increased in strength. It is wound on to reels again at I. Only the first and last two drums are shown. This method was devised to imitate, as far as possible, the sizing process of hand-made paper. Even now, paper that has been made on the machine is sized by hand, after having been cut into sheets, much in the same way as hand-made. [Illustration: THE FOURDRINIER PAPER MACHINE. PLATE II.] [Illustration: Fig. 55.]