_Barking._--The bark on the logs is removed in one or two ways. Much of it is knocked off during the transfer from the forest to the mill, but even then the wood requires to be cleaned. In Norway and Sweden the wood is treated in a _tumbler_ or a _barker_, while in America and Canada the use of the tumbler is practically unknown. The barker consists of a heavy iron disc fitted with knives, usually three in number, which project from the surface of the disc about half or three-quarters of an inch. The barker rotates in a vertical position, and the short pieces of wood are brought one by one into contact with the disc in such a manner that the bark is shaved off by the knives. The machine is provided with conveniences for pressing the wood against the disc and for turning the logs as they are barked. [Illustration: FIG. 28.--A Pair of Barkers for removing Bark from Logs of Wood.] The machine is encased in a strong cast-iron cover, and all the bark shaved off is carried away by the strong current of air set up by the rapid motion of the disc, and subsequently burnt. The tumbler system is quite different. In this case the short pieces are thrown into a large circular drum with hot water, and the bark taken off by the friction of the pieces as the drum rotates. The loss of material is of course less in this process, but the wood is not cleaned quite so effectively. [Illustration: FIG. 29.--View of Horizontal Grinder (A), with Section (B).] The wood at this stage can be used either for the manufacture of mechanical or chemical pulp. As a general rule the pieces are taken indiscriminately for either process, but sometimes the wood is sorted out, the clean stuff free from knots and blemishes being reserved for high quality chemical pulp. _Grinding._--The main feature of the grinding process is the attrition of the wood when held against the surface of a rapidly revolving grindstone, the fibres as they are rubbed off being instantly carried away from the stone by a current of water. A complete description of the machines used and the modifications of the process practised by manufacturers is impossible in this book, but the following points will be sufficient. The machine consists of a large grindstone about 54 inches in diameter, and 27 inches thick. It rotates in a vertical or in a horizontal position at a high speed. The stone revolves inside a casing which is provided with a number of _pockets_, so called, into which the pieces of wood are thrown at regular intervals, as fast as the wood is ground by the friction of the stone. A continual stream of water playing upon the surface of the stone washes away the pulp into a tank or pit below the machine.