"From a fourth-pair window": what is fourth-pair?

The first section of William Henley's poem "Notes on the Firth" (1875) is named "From a fourth-pair window".

What is (а?) fourth-pair, I wonder? A horse-driven carriage of some kind? Or is it an adjective describing the position of the window?

I. — FROM A FOURTH-PAIR WINDOW.

The sky is dappled blue with clouds that stray.
   Like frozen waves the roofs go rolling down
   The valley steeps, but weatherworn and brown
Steeple and stack shoot mastlike toward the day.

Pandean pipes whereon the winds would play,
   Long rows of chimney-pots the ridges crown;
   And black on slates and skylights flicker and frown
Shadows of smoke that streams and wings that sway.

The city's monstrous voices surge to me,
   The mist afar its fantasies arranges,
And sudden windows twinkle joyously.

A blue grey streak, a fixed uncertainty,
   A fallen slip of sky that shifts and changes,
The Forth beyond them broadens into sea.

P.S. Here is the scanned image of the printed text of the poem at Wikimedia.

We turn to OED...

pair

II. A set not limited to two in number.

6. A set of separate things or parts collectively forming a whole, as a set of clothes, a pack of cards, a chest of drawers, etc. Now chiefly Brit. regional and Irish English (north.)
_{Sc. National Dict. s.v. records this sense as still in use with reference to cards in Selkirkshire in 1965.}

9.
a. A set or flight of stairs or steps; (also) a portable set of steps. Also fig.
_{The more recent use of pair of steps to denote a portable set of steps consisting of two joined halves aligns it with the senses in branch I. (see esp. sense 3).}

b. Used with a preceding cardinal (or occas. ordinal) numeral to denote a window or (later usually) room located the stated number of flights of stairs above ground level (freq. qualified by front or back to indicate position within a building). Freq. attrib., as two pair (of stairs) room, etc. Now rare.

Under 9b they give some relevant citations, which actually include fourth pair:

1836 Dickens Let. ?24 Aug. (1965) I. 170 His notion of the Bedroom is rather more derived,..from his own fourth pair back.
1843 Dickens Martin Chuzzlewit (1844) ii. 11 Mr. Pecksniff..turned him loose in a spacious room on the two-pair front.
1853 C. M. Smith Curiosities London Life 360 The little two-pair back-room where we now sit.

So fourth-pair refers to the number of flights of stairs taken to get there: four puts the window on the fourth floor.

What is not obvious at all is the use of pair to refer to something which consists of more than two items. 9b acknowledges the use as rare; 6 now limits it principally to Scotland and Ireland, although it seems to have been more widely used in the mid-nineteenth century when Henley was writing. Henley was in hospital in Edinburgh for three years and discharged in 1875 — possibly to live in the garret he describes — and married a Scot in 1878, so he would have been familiar with Scots English.

Good question.

I first looked for other instances of "fourth-pair window" online, but drew a complete blank.

I then Googled the poem, but there are less than ten sources for it to be found among the resulting hits (at least at my time/space coordinates); and of those few, most seem to be duplicates (specifically, of an OCR'd version of the poem apparently taken from Littell's Living Age/Volume 127/Issue 1644).

Next, I took down from my bookshelf a couple of old poetry anthologies dating from 1932 and 1936. Though they both contained some poems by William Ernest Henley, this one was not among them.

For good measure, I also looked up pair in my copy of Chambers Concise Scots Dictionary (since the location of the house in question is probably Edinburgh, the only town of substantial size that overlooks the Firth of Forth), but found no connotation in Scots that had any relevance to windows. (Edinburgh is also built on a series of steep hills, which would account for "Like frozen waves the roofs go rolling down / The valley steeps".)

The reason for my quest for an original paper-based version of the poem -- or a photographic scan of one -- was to enable me to eliminate the possibility that "fourth-pair window" was an OCR-generated error; a reasonable suspicion, given that no other instance of the term can be found in Google's compendious archives.

(Nor, for that matter, did I encounter any Google hits for "first-pair window", "second-pair window" or "third-pair window".)

All this circumstantial evidence relating to the source of the text causes me to suspect that "fourth-pair window" is actually an OCR mistranscription of "fourth-floor window" **.

Within the poem itself, there are further grounds for supposing that Henley's wording was actually "fourth-floor window". Specifically, the narrator is high up enough to be able to look across chimneys, roof slates and roof ridges, and to glimpse the estuary of the Firth of Forth. In 1875, the technology for constructing buildings around steel frames had not yet been developed, so five storeys was then about as high as you could go.

An answer will be at least one step closer once a genuine printed text of the poem is available; it will dispose of the OCR issue, and if my surmise about a transcription error is correct, it will actually answer your question as well.

If it turns out that the original text really does say "fourth-pair window", then your quest for an answer will have to continue in a different direction.

** This would be known in the US as a fifth-floor window, owing to the different way the storeys of a house are numbered there; in Britain, the customary progression is ground floor, first floor, second floor etc., whereas in the USA it is ground floor (or first floor), second floor, third floor etc.