_Duplex Boards._--If the single board machine is fitted with two vats instead of one, it is possible to manufacture a board with different coloured surfaces. A board coloured red on one side and white on the other is manufactured by having one vat full of pulp coloured red and the second vat full of white pulp. The thin moist sheets from the two vats are brought together and passed through the glazing rolls, which cause the moist sheets to adhere closely to one another, the double sheet of pulp so formed being wound up on the rollers at the end of the machine. The board is then dried, glazed, and finished in the usual way. The same principle is occasionally adopted on the Fourdrinier machine for duplex wrappers. Thus a common brown pulp is worked up in conjunction with a dyed pulp to produce a brown paper having one surface of good paper suitably coloured. The brown pulp flows on to the wire of the paper machine, and after it has been deprived of part of the water at the suction boxes, a thin stream of coloured pulp, diluted to a proper consistency, flows from a shallow trough, placed across and above the wire, on to the wet brown web of paper in such a manner as to completely cover it as a thin even sheet of coloured pulp. The adhesion of the latter to the surface of the brown paper is practically perfect, and the weight of the couch and press rolls ensures uniform felting of the fibres. _Middles._--This term is applied to a thin or thick cardboard made of common material, the colour and appearance of which is of little importance for inferior goods. Boards of this kind are covered subsequently with papers of all colours and qualities, and the origin of the word "middle" is easily seen. The manufacture of a board consisting of two outside papers of good material and a middle produced from common stuff is effected by the continuous boxboard machine, unless the board is too thick to be passed over drying cylinders, calendered, and reeled, in which case the boards are produced on an ordinary wet machine and the paper pasted on the surface of the dry board. The term is, however, now also applied to a common paper made of mechanical wood pulp with perhaps a little chemical pulp, used for tram tickets, cheap advertising circulars, common calendar cards, and similar purposes, to which no outer surface of a special character is added. CONTINUOUS BOARD MACHINE. This machine differs from the single board machine in that the finished board can be produced from the pulp at one operation. It is used principally for cards and boards of moderate thickness which can be wound up in the form of a reel at the end of the machine.