This is one of Coleridge's many fragmentary pieces of poetry, originally written in his notebook in March 1810 [it's Notebooks 3:1 3732, in case you want to track it down], only posthumously included in collections of his complete verse:
When Hope but made Tranquillity be felt—That's the whole poem. The sentiment here rather looks like an earlier version of Dickinson's more famous 'hope is the thing with feathers'. Birdlike hopes flock and flutter around hot, fretful STC, fanning his brow and bringing him tranquility.
A Flight of Hopes for ever on the wing
But made Tranquillity a conscious Thing—
And wheeling round and round in sportive coil
Fann'd the calm air upon the brow of Toil—
The 'but' is a slightly wrongfooting connective, though. Isn't it? It tugs the meaning in two quite different directions: between 'When Hope simply allowed Tranquillity to be felt' on the one hand, and 'When Hope,—though it made Tranquillity be felt ...' on the other. The latter case foregrounds the fragmentariness of the fragment, and would invite us to take the whole text from 'but made...' through to '...of Toil' as a subordinate clause, to be followed by some kind of negating main verb. For example: 'When hope—though it made tranquillity be felt—failed ...' and so on. The first reading is the one most people would gravitate towards, I suppose; although even if we take this short piece as a little moment of optimism the repetition, 'but ... but ...' nags, rather, at the whole.
I wonder if Coleridge's prompt for this fragment was Francis Bacon's essay 'Of Hope' (it was number 13 in the second section of the final collected essays, ‘Essays on Oeconomical Subjects’). Perhaps counter-intuitively, Bacon counsels against hope. 'Hope and Fear are two bad Presagers, and not to be trusted,' he says, adding:
If the Success prove less than was hoped, it seems to be rather a Loss than a Gain; as falling short of the thing expected. If the Success be adequate to the Hope; yet even thus the Flower of the Benefit is cropped by Hope; and fades in the Enjoyment. Lastly, If the Success be greater than was hoped, there seems indeed to be some Advantage received; but were it not still better to have had the Principal without hoping at all, than the Interest, by hoping too little? And this is the Operation of Hope in Matters of Prosperity. In Matters of Adversity, Hope breaks the true Courage of the Mind, for there is not always an Occasion of hoping; and with any, even the least Desertion of Hope, almost the whole Support of the Mind is gone. It also lessens our Dignity to bear Misfortunes by a certain Alienation and Error of the Mind; and not with Firmness and Strength of Judgment. 'Twas therefore wrong in the Poets to make Hope the Antidote and Mitigator of human Calamities, when in reality it rather exasperates, multiplies, and renews them.Bacon then considers, and dismisses, one possible counter-argument:
It may be asked, if it not better, since Things are placed in Uncertainty, to expect the best, and rather to hope than despair; because Hope procures the greater Tranquillity to the Mind? I answer, that in all Delay and Expectation, I judge a serene and steddy State of the Mind, arising from a due Regulation and Composure thereof, to be the principal Strength and Support of human Life; but reject the Tranquillity which depends upon Hope, as a weak Thing. Tho it may not be improper, from a sound and sober Conjecture, to presuppose and foresee both good and bad Fortune, that we may the better suit our Actions to the Probability of Events; provided this be made the Office of the Understanding and Judgment, and is attended with a just Sway of the Affections.We know Coleridge read Bacon; we know that he often despaired and craved a sense of individual and spiritual tranquility that receded ever before him as he moved. Might this last quoted paragraph of Bacon's have provided the substance and meat of the latter portion of this poem, had Coleridge ever completed it? Coleridge's Baconian old-school spelling of 'tranquillity' (a word much more often spelled 'tranquility' by 1810) makes me think so.