Adam Clarke on Ecclesiastes 1:7

All the rivers run into the sea; yet the sea is not full - The reason is, nothing goes into it either by the; rivers or by rain, that does not come from it: and to the place whence the rivers come, whether from the sea originally by evaporation, or immediately by rain, thither they return again; for the water exhaled from the sea by evaporation is collected in the clouds, and in rain, etc., falls upon the tops of the mountains; and, filtered through their fissures, produce streams, several of which uniting, make rivers, which flow into the sea. The water is again evaporated by the sun; the vapors collected are precipitated; and, being filtered through the earth, become streams, etc., as before.