Thoreau mostly uses imagery to illustrate time in the excerpt. The second sentence is saying that he is in the midst of time/living, and he can "detect how shallow it is," meaning that he is aware that life is short and that the end is inevitable. Thoreau also describes time as being fleeting, but ever-present.
I hope this helps!
The answers "ideas are learned". The condition of being enlightened: to live in profound edification. The Enlightenment, a philosophical development of the eighteenth century, portrayed by confidence in the energy of human reason and by advancements in religious, political, and instructive regulation.
