Hosptial Barge At Cerisy

I learnt this as part of my A Level English and have never forgotten it....especially the last three lines.

Budging the sluggard ripples of the Somme,
A barge round old Cérisy slowly slewed.
Softly her engines down the current screwed
And chuckled in her, wtih contented hum.

Till fairy tinklings struck their croonings dumb.
The waters rumpling at the stern subdued.
The lock-gate took her bulging amplitude.

Gently from out the gurgling lock she swum.
One reading by that sunset raised his eyes
To watch her lessening westward, quietly,
Till, as she neared the bend, her funnel screamed.
And that long lamentation made him wise
How unto Avalon, in agony,
Kings passed, in the dark barge, which Merlin dreamed