A sentence that opens with "When" and never says what happens next is a strange thing to meet. That is the situation with these five lines by Coleridge, which break off before the main clause arrives. We are given no date and no account of why the piece stopped, so it is safest to treat the fragment as it stands: a small, complete image inside an unfinished thought.
The idea at its centre is modest and a little surprising. Hope, we are told, "but made Tranquillity be felt." The word "but" matters. Hope does not deliver peace or solve anything; it only makes calm perceptible. Tranquillity here is not an absence of trouble but something that has to be noticed, and hope is what lets us notice it. The second line turns that single Hope into "A Flight of Hopes for ever on the wing," as though one feeling had broken into a flock of birds.
The construction is neat. Every line has ten syllables in a mostly regular iambic pattern, which gives the fragment a smooth, almost eighteenth-century courtesy. The rhymes run felt / wing–Thing / coil–Toil. The first line has no partner, while the next four form two couplets. That orphaned opening is a clue that something came before or was meant to follow, though that is an inference from the pattern, not a known fact. The couplets themselves click shut with confidence, which makes the missing main clause more noticeable.
The imagery works through two pictures laid over each other. The hopes are birds: they are "on the wing," "wheeling round and round." But by the last line their beating wings have become a fan, as they "Fann'd the calm air upon the brow of Toil." Toil is personified as a labourer with a sweating forehead, and hope is the cooling movement across it. The phrase "sportive coil" is especially good. A coil suggests something wound tight, even a spring or a snake, yet "sportive" makes it playful. The rhyme of "coil" with "Toil" quietly joins play and labour, so that the hopes' frolic turns out to serve the worker.
Then there is the paradox of motion producing stillness. The hopes never settle ("for ever on the wing") and circle "round and round," yet the result is calm. Even "the calm air" is already calm before it is fanned, so the hopes do not create peace so much as stir it into being felt. The repeated words round and round copy the circling, and line 3 circles back too, restating line 1 almost word for word: "made Tranquillity be felt" becomes "made Tranquillity a conscious Thing." That shift is the most intriguing moment. Tranquillity moves from something felt to something conscious, nearly a being with awareness of itself. The capital letters on Hope, Tranquillity, Flight and Thing belong to the old habit of personifying abstractions, and here they make calm feel like a presence that wakes up in the room.
What makes the fragment worth reading is how much it packs into five lines. It offers a small, unsentimental view of hope: not a promise that things will turn out well, but a gentle draught of air for someone in the middle of hard work. The form seems to agree with the content. Hope is always "on the wing," always forward-looking, never landing, and the sentence never lands either. Whether or not that was intended, the unfinished grammar makes the reader feel what the lines describe, a thought still in flight.