284	BOOK VIII.
	can remove with a shovel the fine sand, and likewise the coarse sand and broken rock, into which the rocks have been crushed ; this board can be lowered, so that the mouth thus being closed, the fresh rock thrown in may be crushed with the iron-shod stamps. If an oak block is not available, two timbers are placed on the ground and joined together with iron clamps, each of the timbers being six feet long, a foot wide, and a foot and a half thick. Such depth as should be allowed to the mortar, is obtained by cutting out the first beam to a width of three-quarters of a foot and to a length of two and a third and one twenty-fourth of a foot. In the bottom of the part thus dug out, there should be laid a very hard rock, a foot thick and three-quarters of a foot wide ; about it, if any space remains, earth or sand should be filled in and pounded. On the front, this bed rock is covered with a plank ; this rock when it has been broken, should be taken away and replaced by another. A smaller mortar having room for only three stamps may also be made in the same manner. The stamp-stems are made of small square timbers nine feet long and half a foot wide each way. The iron head of each is made in the following