I wish I could help but it makes no sense to me :(((
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!
Answer:
A professional diver does a belly flop is the best example. Hope that helps!
