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.
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Answer: Because when Sir Gawain cut off the nights head, the head and reminded him the pact that Gawain had to seek him in a year and a day at the Green Chapel.
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A portion of a word that forms the base of a word is its Baseword