What metaphor does Thoreau us for time? What comparison is he making?

I can pick out two metaphors for time. The first one is, “Time is but the stream I go fishing in.” This represents the eternity of time and we are but a moment in that endless flow. The other metaphor I might consider is “I have always been regretting that I was not as wise as the day I was born,” This represents the innocence and happiness of childhood as opposed to to the complex stress of adulthood.