During the war, however, the necessity to exploit the forests of Britain to contribute to the requirements of the army and mines in regard to wood has resulted in the piling-up of huge heaps of sawdust. It was discovered that in Scotland alone this residue was accumulating at the rate of 60,000 tons a year, through the activity of the Canadian lumberjacks. Conservative estimates place the annual sawdust yield throughout the British Isles at 150,000 tons. Of this gigantic contribution only from 5 to 10 per cent. is drawn from hard woods. The balance, 90 to 95 per cent., is derived from the soft woods and so furnishes a huge reservoir of potential raw material for paper-making. Coincident with the accumulation of sawdust are the fabrication of huge piles of wood-slats--the trimmings from the logs. These also represent sheer refuse, the only possible disposal being in the form of fire or kindling wood. At one lumber-camp in Scotland there was found a pile, a sprawling, ragged and jagged stack, house-high, covering 20 acres, and containing, at a modest estimate, from 300 to 500 tons of wood-waste. It was ideal for paper-making as investigations proved, but was then merely being allowed to rot. The process of preparing sawdust for the paper-maker is very simple and inexpensive. It may be described as an application of the system for producing mechanical pulp, because, in the main, the resultant product is very similar to the latter in its essential characteristics. The waste, being the product of the buzz-saw, is coarse in texture. It is first passed over a riddle of wide mesh, which, while allowing the dust proper to fall through readily, collects the pieces of bark, chips, and other fragments of wood which may have become associated with the dust. This residue is thrown to one side for conversion by a different method. The sifted sawdust is dumped into a hopper to fall by gravity in a steady stream into the mill, which is somewhat reminiscent of the familiar mortar-mill, below. As it enters the latter it is caught up by the revolving grindstone and crushed against the stationary stone, the result being that it is disintegrated and pulverized. By virtue of the centrifugal action set up the dust, as it is whirled round, naturally works from the centre to the periphery of the wheels, the coarser particles or tailings being flung out, while the finely-divided dust, produced by the grinding action, falls into a separate receptacle.