The fluid pulp enters the machine through a flow box, where a constant hydraulic head is maintained, so as to preserve a uniform discharge to the wire gauze. The milky stream issues through a narrow opening in the flow box, known as a “slicer.” This may be adjusted to alter the width and also the thickness of the stream. No one would suspect from the appearance of the milky fluid that it is filled with fibers, but a microscopic examination of a drop of the thin liquid shows that it is filled with tiny particles floating in the water like logs in a river. It is interesting to watch this stream, as it passes out upon the screen, change magically before one’s eyes, from a milky stream into solid mass of pulp. [Illustration: FIG. 71.--DIAGRAMMATIC VIEW OF THE FOURDRINIER PAPER-MAKING MACHINE A. Flow Box B. Copper Gauze Belt C. Rollers D. Deckles Confining Pulp EE. Pulleys FF. Drums G. Suction Boxes H. Dandy Roll I. The Final Suction Box for Drying J. Couch Roll K. Canvas Belt ] In Figure 71, which is a diagrammatical view of a portion of a Fourdrinier machine, the flow box is shown at _A_ and the copper gauze belt at _B_. This belt is supported on a series of rollers, _C_. The deckles which confine the pulp along the margins of the screen are shown at _D_. These belts travel with the wire gauze for a certain distance and then pass back over the pulleys, _EE_, while the wire cloth passes on and around the drums, _FF_. As soon as the pulp flows out upon the wire gauze, the water it contains begins to drain away through the gauze. But it does not do this fast enough under the mere pull of gravity and it must be helped by means of suction. At _GG_ there are boxes or troughs that fit against the under side of the wire gauze. In them a partial vacuum is maintained by means of suction pumps and a large part of the water remaining in the pulp is sucked out here. A dandy roll is shown at _H_, which presses lightly upon the moist pulp, squeezing out the moisture in it, and at _I_ there is a final suction box which further dries the web of fibers. It is the dandy roll that puts a patterned surface on the paper. If the face of the dandy roll is of finely woven wire cloth with diagonal mesh, a “wove” paper is produced, while if it is of such surface as to impress parallel lines in the half-formed paper, “laid” paper is produced. Water marks are also produced by placing a suitable raised or depressed pattern on the dandy roll, producing a corresponding thickening or thinning of the pulp. Even after passing the last suction box, the thin web of paper is still very wet and so flimsy that it cannot be handled. Right here comes the most delicate part of the whole process. The web must be picked off the wire cloth and carried across a narrow gap to a felt belt which conducts it through another set of rollers, A felt-covered roll, _J_, known as a “couch” roll, presses down upon