Dusk on an empty beach, a path of light across the water, and then, from nowhere, a bawdy sailors' song: Edward Thomas builds «An Old Song» from these three things and lets the last one take over.
The opening stanza works by subtraction. "The sun set, the wind fell, the sea / Was like a mirror shaking": each short clause switches something off, and the sea is left in a strange state, still enough to be a mirror yet trembling. Against that hush, sound is small and exact. One wave "clapped the land," a word that makes a tiny splash sound like applause, and leaves behind "a mile-long snake of foam." The stanza closes on its shortest line, "The vacant sand," so the page itself seems to empty out. Rhyme is used sparingly. Only "shaking" and "making," and "land" and "sand," pair up, so the stanza feels loosely tied, like the tide-smoothed beach it describes.
In the second stanza the scene becomes personal. A light "divided the swollen clouds" and lies "like a straight narrow footbridge bright / That crossed over the sea to me." The comparison is homely. It is a plain footbridge, not a golden road or a stairway to heaven, and that makes the vision more believable. The path runs to "me," which puts the speaker at the centre of the evening. The stanza ends in an almost childlike boast, tinged with loneliness: "no one else in the whole world / Saw that same sight." There is a quiet echo here too. "Perfectly" and "me" rhyme with "sea" from the first stanza, as if the water and the watcher were being tied together by sound.
Then the poem turns on a small, frustrated joke. "I walked elate, my bridge always / Just one step from my feet." The bridge travels as he does, so it can be looked at but never stepped onto. Beside that unreachable brightness the robin is "a shade in shade," heard rather than seen, which tells us the light is going. Sight is giving way to sound. The speaker also stops describing: "all I did was to repeat" words that are not his own. The refrain "I'll go no more a-roving / With you, fair maid" arrives here, and from this point it grows through the poem like a tune that won't leave the head.
The fourth stanza explains where the words come from, with a candour that is rather disarming. This is a "sailors' song of merry loving," and the speaker admits its "lewdness," but says the "wild charm the chorus played" outweighs it. The song is "Mixed sweet" with "dusk and sea-gull's mewing," so it becomes part of the landscape, one more evening sound. The refrain also expands, from two lines to the full chorus with its "a-roving, a-roving, since roving's been my ruin." In the final stanza, printed in italics, the speaker disappears altogether. We get only the song's own verse: "In Amsterdam there dwelt a maid," repeated, then "Mark well what I do say," a little ironic when what follows is nothing weighty. The poem ends inside the chorus, with no comment or conclusion.
What the poem seems to be about is the gap between an experience and the words available for it. The vision on the water is too private and too delicate to explain, so the mind reaches for something ready-made. The irony is pointed. The sailor swears off roving, yet the man singing it is walking alone along the shore after a bridge that always stays a step ahead. Judging by the imagery, the poem suggests that the pull of the unreachable is exactly what the old song cannot cancel.
That is also why the poem matters. It moves from "no one else … saw that same sight" to a song that countless voices have sung. The solitary moment is not explained but joined to something shared and anonymous, and the plain language, short lines and unfinished feeling let the reader hear that happen.