what in the second paragraph sets the mood for the story?

If we have the same second paragraph (old texts can be different), it goes like this:

"The sea-reach of the Thames stretched before us like the beginning of an interminable waterway. In the offing the sea and the sky were welded together without a joint, and in the luminous space the tanned sails of the barges drifting up with the tide seemed to stand still in red clusters of canvas sharply peaked, with gleams of varnished sprits. A haze rested on the low shores that ran out to sea in vanishing flatness. The air was dark above Gravesend, and farther back still seemed condensed into a mournful gloom, brooding motionless over the biggest, and the greatest, town on earth."

There is a sense of exploration, as if it was somehow God's calling to man,

"the sea and the sky were welded together without a joint, and in the luminous space the tanned sails of the barges drifting up with the tide..."

There is however a sense of foreboding as well," a mournful gloom, brooding motionless ." This really sets the mood and tone for the rest of the book. While Marlow is enraptured by exploration, the novel gets darker and darker as he drifts into the heart of the Congo.