Visions: "I saw a silver swan"

I saw a silver swan swim down the Lea,
Singing a sad farewell unto the vale,
While fishes leapt to hear her melody,
And on each thorn a gentle nightingale
And many other birds forbore their notes,
Leaping from tree to tree, as she along
The panting bosom of the current floats,
Rapt with the music of her dying song:
When from a thick and all-entangled spring
A neatherd rude came with no small ado,
Dreading an ill presage to hear her sing,
And quickly struck her tender neck in two;
Whereat the birds, methought, flew thence with speed,
And inly griev'd for such a cruel deed.