When the dust groweth into hardness - Margin, “is poured, or, is turned into mire.” The words used here relate often to metals, and to the act of pouring them out when fused, for the purpose of casting. The proper idea here is, “when the dust flows into a molten mass;” that is, when wet with rain it flows together and becomes hard. The sense is, that the rain operates on the clay as heat does on metals, and that when it is dissolved it flows together and thus becomes a solid mass. The object is to compare the effect of rain with the usual effect in casting metals.
And the clods cleave fast together - That is, they are run together by the rain. They form one mass of the same consistency, and then are baked hard by the sun.