By WB Yeats (biography in Wikipedia)

The trees are in their autumn beauty,
The woodland paths are dry,
Under the October twilight the water
Mirrors a still sky;
Upon the brimming water among the stones             5
Are nine and fifty swans.

The nineteenth Autumn has come upon me
Since I first made my count;
I saw, before I had well finished,
All suddenly mount      10
And scatter wheeling in great broken rings
Upon their clamorous wings.

I have looked upon those brilliant creatures,
And now my heart is sore.
All’s changed since I, hearing at twilight,      15
The first time on this shore,
The bell-beat of their wings above my head,
Trod with a lighter tread.

Unwearied still, lover by lover,
They paddle in the cold,      20
Companionable streams or climb the air;
Their hearts have not grown old;
Passion or conquest, wander where they will,
Attend upon them still.

But now they drift on the still water      25
Mysterious, beautiful;
Among what rushes will they build,
By what lake’s edge or pool
Delight men’s eyes, when I awake some day
To find they have flown away?      30