An anapestic foot (known as an anapest) has two short syllables followed by a long syllable (SSL, or UU/).
Trimeter is three feet per line.
Not many poems are written entirely in anapestic trimeter, but this one – about a sailor living on a deserted island after jumping ship – comes very close.
Note that some lines – such as “My right there is none to dispute” and “O Solitude! where are the charms” – actually begin with iambs (SL, or U/). This helps avoid the monotony of continual anapests, giving occasional added emphasis.
I am monarch of all I survey;
My right there is none to dispute;
From the centre all round to the sea
I am lord of the fowl and the brute.
O Solitude! where are the charms
That sages have seen in thy face?
Better dwell in the midst of alarms,
Than reign in this horrible place.
I am out of humanity's reach,
I must finish my journey alone,
Never hear the sweet music of speech;
I start at the sound of my own.
The beasts that roam over the plain
My form with indifference see;
They are so unacquainted with man,
Their tameness is shocking to me.
William Cowper – from The Solitude of Alexander Selkirk
A closer look at the feet in this poem
U = short syllable; / = long syllable; | = division between feet