Psalms 104:10

He sendeth the springs into the valleys - Evaporation is guided and regulated by Divine Providence. The sun has a certain power to raise a certain portion of vapours from a given space. God has apportioned the aqueous to the terrene surface, and the solar attraction to both. There is just as much aqueous surface as affords a sufficiency of vapours to be raised by the solar attraction to water the earthy surface. Experiments have been instituted which prove that it requires a given space of aqueous surface to provide vapours for a given space of terrene surface; and the proportion appears ordinarily to be seventeen of water to three of earth; and this is the proportion that the aqueous bears to the terrene surface of the globe. See Ray's three Physico-theological Discourses.

He sendeth the springs into the valleys - Though the waters are gathered together into seas, yet God has taken care that the earth shall not be dry, parched, and barren. He has made provision for watering it, and by a most wise, wonderful, and benevolent arrangement, he has formed springs among the valleys and the hills. It is now animated nature which comes before the eye of the psalmist; and all this he traces to the fact that the earth is "watered," and that it is not a waste of rocks and sands. The allusion in this part of the psalm (see the Introduction) is to the earth as covered with vegetation - or, to the third day of the week of creation Genesis 1:9-13, which, in Genesis, is connected with the gathering of the waters into seas. This description continues in Psalm 104:18. The literal rendering here would be, "sending springs into the valleys." He conducts the waters from the great reservoirs - lakes and seas - in such a way that they form springs in the valleys. The way in which this is done is among the most wonderful and the most benevolent in nature - by that power, derived from heat, by which the waters of the ocean, contrary to the natural law of gravitation, are lifted up in small particles - in vapor - and carried by the clouds where they are needed, and let fall upon the earth, to water the plants, and to form fountains, rivulets, and streams - and borne thus to the highest mountains, to be filtered through the ground to form springs and streams below.

Which run among the hills - Margin, "walk." That is, they go between the hills. The streams of water flow along in the natural valleys which have been made for them.