Jamieson, Fausset and Brown on Ecclesiastes 1:7

7. By subterraneous cavities, and by evaporation forming rain clouds, the fountains and rivers are supplied from the sea, into which they then flow back. The connection is: Individual men are continually changing, while the succession of the race continues; just as the sun, wind, and rivers are ever shifting about, while the cycle in which they move is invariable; they return to the point whence they set out. Hence is man, as in these objects of nature which are his analogue, with all the seeming changes "there is no new thing" (Ec 1:9).