202 ON BUILDING MILL-WALLS. [Chap. 8.

3. Use good mortar, and it will, in time, become ashard as stone.*

4. Arch over all the windows, doors, &c.

5. Tie them well together by the timbers of the floors.

* Good mortar, made of pure, well-burnt limestone, properly made up withsharp, clean sand, free from any sort of earth, loam, or mud, will, in time, actuallypetrify, and turn to the consistence of a stone. It is better to put too much sandinto your mortar than too little. Workmen choose their mortar rich, because itworks pleasantly; but rieh mortar will not stand the weather so well, nor grow sohard as poor mortar. If it were all lime, it would have little more strength thanclay.