The narrator describes the recent history of the Thames. What are some of the activities of the men who have sailed out, from the mouth of the river?

Basically the narrator pays tribute to the thousands of British heroes who brought wealth and civilization to the savages in other lands,

"Hunters for gold or pursuers of fame, they all had gone out on that stream, bearing the sword, and often the torch, messengers of the might within the land, bearers of a spark from the sacred fire. What greatness had not floated on the ebb of that river into the mystery of an unknown earth! . . . The dreams of men, the seed of commonwealths, the germs of empires."

Once we finish the book we understand there is sarcasm here!