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!
You can taste the freshness of the air aswell as the moss of the ground if you feel like it, i mean honestly everything you can taste if you dare to stick it in your mouth
Dilemma: having to make a difficult decision between (usually 2) options, “I was in a dilemma because I couldn’t decide whether to snack now or eat a big dinner later.”, problem, predicament, difficulty
feebly: in a way that is weak or lacks strength, “My grandma feebly walked into the kitchen.”, awkwardly, clumsily.
persistent: to be dedicated to doing something and trying repeatedly, “The toddler was persistent while trying to stand up.”, dedicated, constancy
recoiled: to move back in fear, disgust, or horror, “She recoiled at the sight of the bug.”, flinch, wince
roused: to be woken up, “He roused from his nap after a couple hours.”, awaken, stimulate
skewed: to move out of tune, “Their face skewed as they began to cry.”, squint, slope, asymmetrical
summon: to bring someone to you urgently, “He summoned their dog with a treat.”, invite, call for
vastness: something of a large size (usually empty), “She looked upon the vast ocean at the beach.”, empty, big
hope these helped!! good luck with school and remember to take care of yourself :)