The last stanza of the poem “The Ecstatic” contains a metaphor comparing the passing day to a tide. The entire poem is a metaphor for enjoying life while it lasts.
A metaphor is a comparison used for descriptive effect. In literature, a metaphor often uses a thing to “represent a less tangible thing or some intangible quality or idea” (enotes topic, metaphor). For instance, there is a metaphor in the first stanza of the poem.
…spilling your rubbed and round
Pebbles of sounds in air’s still lake (lines 1-2)
Here, the sound of the bird calling in the air is compared to pebbles making rings in a pond. The reader thus pictures sound ringing through the air. This is not a simile because it does not say “like pebbles in a pond.”
In the last stanza, there is another metaphor. The speaker asks the bird to be strong in the second stanza, because “soon Light’s tide will turn” (line 10). This metaphor compares the change from day to night to the tide coming in and out.
Cease not till day streams to the west, then down
That estuary drop down to peace (line 11-12)
This is a particularly fitting metaphor for a bird. In this poem, the light is compared to the tide because birds need water. If you look at the larger metaphor, the poem is really about enjoying life and making the most of it while you can because life is so brief.
Thus, there are two layers of metaphor here. On the one hand, the passing day is compared to a wave, and on the other hand life is compared to a soaring bird. The brevity of life is represented by the change in day to night, and the joy of life is represented by the bird’s song.