This passage narrates Satan’s perilous journey through the landscape of Chaos towards Eden. Milton’s allusions to classical mythology make the scene more vivid: the quicksands of Syrtis and the pursuit of the gryphon—a creature with the body of a lion but the wings and head of an eagle—point to the strange mixture of movements that Satan has to employ to traverse an unfamiliar and restrictive environment. The ‘boggy’ atmosphere of this passage is most richly expressed, however, in effects of the poem’s metre: the journey is as challengingly uneven for us readers as it is for Satan.

Paradise Lost is written in iambic pentameter (the same as much of Shakespeare’s works, or the first line of ‘Paperback Writer’ by the Beatles), a poetic metre that is made up of five ‘feet’ of iambs per line of poetry. An iamb comprises one unstressed syllable followed by a stressed one, such as the word ‘again’, and Paradise Lost rolls on in this steady measure, except for countless metrical derivations ranging from the minor to the outrageous.

In fact, it would have been much more unusual for Milton to write a poem that entirely conformed to the strictest definition of his chosen metre than it is for him to bend the rules. One of the most common ways to spice up iambic pentameter is to change the first syllable of a line from an unstressed syllable to a stressed one, like ‘Quenched’ at the beginning of our passage. This can inject some energy into a line, but near the end of our passage stressed syllables accumulate until they become suffocating: 

So eagerly the fiend
O'er bog or steep, through strait, rough, dense, or rare,
With head, hands, wings, or feet pursues his way...

We do not need an advanced vocabulary of poetic analysis to say that these lines, describing intensely restricted movement, feel stodgy and dense to read. Looking more closely at this effect might lead us to investigate other kinds of poetic feet—trochees, spondees, and their ilk—that are lurking around here, or we might consider how it is actually an abundance of commas that supports these insistent feet: the hard-won flow of ‘pursues his way’ stems in part from a relaxation of punctuation that matches Satan’s gradual escape from the mire of Chaos.

We can consider this irregularity with the help of another poet: T.S. Eliot trumpets that Milton is ‘is the greatest master in our language of freedom within form’, and states that Paradise Lost ‘is continuously animated by the departure from, and return to, the regular measure’. Here is more language of constraint, freedom and traversal; maybe writing and reading poetic metre is always an uncertain journey—one that we make on foot.

The passage’s uneasiness suddenly resolves into a line that Gordon Teskey notes is one of the only ‘perfect’ instances of iambic pentameter in the poem: Satan stubbornly persists, ‘[a]nd swims, or sinks, or wades, or creeps, or flies’. Satan’s flexibility is now answered by an absolutely regular metre; a moment of freedom coincides with the return of poetic constraint. Samuel Johnson famously warned that seeking out a correspondence between ‘sound and sense’ in poetry can produce ‘wild conceits and imaginary beauties’. The presence of deliberate metrical effects in this passage is undeniable, but the details of their workings are as knotty and uncertain as Satan’s own amorphous terrain; we can only try to match his persistence in navigating them.