When it dries, the fly-ash acts as a coating, able to withstand the elements in exposed places.

干燥之后，煤灰会覆盖在表层，能够抵抗日晒雨淋。

Therefore I stationed some of the people behind the lowest points of the wall at the exposed places, posting them by families, with their swords, spears and bows.

所以我使百姓各按宗族拿刀、拿枪、拿弓，站在城墙后边低洼的空处。