John Calvin Commentary Psalms 104:9

"Thou hast set a bound that they may not pass over; That they turn not again to cover the earth." — Psalms 104:9 (ASV)

You have set a bound which they shall not pass. The miracle spoken of is amplified in this verse by its perpetuity. Natural philosophers are compelled to admit—and it is even one of their first principles—that water is circular and occupies the region intermediate between the earth and the air.

It is entirely due to the providence of God that part of the earth remains dry and fit for human habitation. This is a fact for which sailors have the most satisfactory evidence. Indeed, if even the most uncultured and dull-witted among us were only to open their eyes, they would see in the sea mountains of water elevated far above the level of the land.

Certainly, no banks, and not even iron gates, could make the waters—which by their very nature are fluid and unstable—stay together in one place, as we see them do.

I have just said that earthquakes, which bring destruction to some places, leave the globe, on the whole, as it was before. Likewise, although the sea in some parts of the world overflows its boundaries, the law that confines it within certain limits stands firm, so that the earth may be a fit habitation for humans.

The Baltic Sea, in our time, inundated large tracts of land and inflicted great damage on the Flemish people and other neighboring nations. Such an instance warns us what the consequence would be if the restraint imposed on the sea by God's hand were removed.

Why, then, have we not been swallowed up by it, if not because God has restrained that raging element by His word? In short, although the natural tendency of the waters is to cover the earth, this does not happen, because God has established by His word a counteracting law; and since His truth is eternal, this law must remain steadfast.