Preface. [1] In a note...by Mr. W. Wordsworth to a poem.
The poem is "The Thorn", written 1n 1798, the year of some of Wordsworth's best work (We are Seven, Tinturn Abbey, Peter Bell). It begins
"There is a Thorn--it looks so old,
In truth, you'd find it hard to say
How it could ever have been young,
It looks so old and grey....

(The entire text of "The Thorn" and all of Wordsworth's poems , is at Project Bartleby ).

I have not seen the note mentioned by Byron. It is omitted in my edition of the Complete Poems (MacMillan, 1907).

Preface. [2] I measured...two feet wide.
These lines do not appear in my edition of "The Thorn". The pond is described this way (stanza 3):
...to the left, three yards beyond,
You see a little muddy pond
Of water--never dry
Though but of compass small, and bare
To thirsty suns and parching air.
Preface. [3] rustic Gongora
Luis de Góngora (1561-1627) was a Spanish poet who wrote in an extravagant style popular in his day but later considered ridiculous.
Preface. [4] vulgar Marini
Giovanni Marini (1569-1625) was an extravagant Italian poet.
Preface. [5] Thraso...Gnatho
Thraso and Gnatho are two characters in the Eunuchus of Terence. The first is a braggart, the second a hanger-on.
Dedication.2 [1] Explaining metaphsics to the nation...
In his Biographica Literaria (1817).
Dedication.3 [2] a-dry, Bob
In the slang of the time, a "dry Bob" described an incomplete sexual act. A pun this low and obvious probably could not have been included in the material actually published in 1818.
Dedication.6 [3] ...place in the Excise
(Byron's note.) Wordsworth's place may be in the Customs -- it is, I think, in that or the Excise -- besides another at Lord Lonsdale's table, where this poetical charlatan and political parasite licks up the crumbs with a hardened alacrity; the converted Jacobin having long subsided in the the clownish sycophant of the worst prejudices of the aristocracy.
Dedication.13 [4] Ixion's grindstone
Ixion was a king of Thessaly who, by a complicated set of circumstances, was received at the table of the Gods. When he tried to seduce Hera, Zeus had him tied to an eternally-moving wheel in hell.
Dedication.15 [5] Eutropius
A eunuch who rose to great power in the imperial court at Constantinople.
Dedication.17 [6] buff and blue
The colors of the Whig party.
Dedication.17 [7] ultra-Julian
The reference is to the emperor Julianus, or "Julian the Apostate". A nephew of Constantine, he took power after that emperor's death and renounced Christianity, returning to the worship of the ancient gods of Rome.
Canto 1:2 [1] "nine farrow" of that sow
This image, now rather obscure, previews the kind and quality of wordplay found throughout the poem. The "nine farrow", or piglets, are the nine heroes named at the top of the verse. They are pictured parading in a line after their sow, and compared to the line of 8 kings, then Banquo, that the witches make appear to Macbeth (Act IV, scene i). The witch's incantation to begin the parade of apparitions is: Pour in sow's blood, that hath eaten
Her nine farraw; grease that's sweaten
From the murderer's gibbet throw
Into the flame.

This combination of irreverance, learning, and willingness to invoke images based on harsh reality, recurs throughout the poem.

Canto 1:5 [2] Brave men were living before Agamemnon
This line is lifted directly from one of the Hymns of Homer. Agamemnon was of course the great general of the Greeks at Troy.
Canto 1:6 [3] in media res
The Greek herioc epics typically begin "in the middle of things". The canonical example is the Iliad. The Roman Horace, who adhered to the Greek model, declared this device a given.
Canto 1:10 [4] His mother
Although he denied it, Byron put much of his estranged wife in the character of Donna Inez, Don Juan's mother.
Canto 1: 11 [5] Feinagle's...art
George Feinagle taught a system of mnemonics.
Canto 1: 16 [6] female errors fall
See Pope's The Rape of the Lock, Canto II, lines 17-18: If to her share some female errors fall,
Look on her face, and you'll forget them all.
Canto 1: 17 [7] thine "imcomparable oil," Macassar
Oil of Macassar was a hair tonic produced on the French Island of Macassar. Its name survives today in the term "ani-Macassar" still applied to linen covers designed to prevent soiling of furniture by oily hair. Advertisements for this pomade described it as "incomparable."
Canto 1: 21 [8] brain him with their lady's fan
See Henry IV, Part 1, Act II, scene iii, Hotspur's first speech: I could brain him with his lady's fan.
Canto 1: 35 [9] Numa...Pompilius
Numa Pompilius was the second King of Rome, succeeding Romulus. In contrast to Romulus, he was not a man of war, preferring to concentrate on the arts of peace. His supposed 'indescretion' was with the nymph Egreria, in whose name he instituted many innovations in government and religion.
Canto 1: 37 [10] messuages
A legal term meaning houses and the adjoining lands.
Canto 1: 37 [11] An only son left with an only mother
The autobiographical element of the satire here shifts from the relationship of Byron and his wife to that of Byron and his mother.
Canto 1: 42 [12] Formosum Pastor Corydon
This Eclogue deals with the love the shepherd Corydon for the boy Alexis.
Canto 1: 53 [13] Verbum sat
"A word [to the wise] is sufficient."
Canto 1: 55 [14] zone
girdle.
Canto 1: 62 [15] mi vien in mente
"It comes to my mind"
Canto 1: 64 [16] St. Anthony
It was actually St. Francis who was said to have a "wife of snow". Byron meant to change the line, but never did.
Canto 1: 72 [17] Armida
The sorceress in Tasso's Jerusalem Delivered.
Canto 1: 88 [18] O Love!...god indeed divine
These lines open the third canto of Thomas Campbell's Gertrude of Wyoming.
Canto 1: 122 [19] Adria
Venice
Canto 1: 129 [20] One makes new noses
An American quack, Benjamin Charles Perkins, invented a metal "tractor" or attractor for drawing poisons from the body. It was advertised as a cure for "all disorders, Red Noses, Gouty-toes, Windy Bowels, Broken Legs, Hump Backs."
Canto 1: 129 [21] Congreve's rockets
William Congrever was the inventor of a sort of guided rocket first used at the Battle of Leipzig (1813). According to Pratt's note, "Although it did little actual damage, the noise and the bright glare brightened the French and threw them into confusion."
Canto 1: 130 [22] Humane Society
American readers might mistake this for the SPCA. In fact, the Humane Society was founded in 1774 for the rescue of drowning persons.
Canto 1: 130 [23] the great [pox]
Syphilis
Canto 1: 132 [24] Sir Humphrey Davy's lantern
Davy, the well-known chemist, invented the miner's safety lamp. He and Byron were friends.
Canto 1: 148 [25] Cortejo
Byron's note: "The Spanish `Cortejo' is much the same as the Italian `Cavalier Servente." And a "Cavalier Servente" was a kind of lady's gentleman, a male companion-servant to a married woman.
Canto 1: 148: [26] General Count O'Reilly
Byron's note: "Donna Julia here made a mistake. Count O'Reilly did not take Algiers--but Algiers very nearly took him: he and his army and fleet retreated with great loss, and not much credit, from before that city...." Alexander O'Reilly (172?-1794), born in Ireland, rose to great rank in Spain, governing Madrid and Cadiz. He led the unfortunate expedition against Algiers in 1775.
Canto 1: 162: [27] Alfonso saw his wife, and thought of Job's
Pratt's note refers to this verse: "Thou speakest as one of the foolish women speaketh." -- Job 2:10.
Canto 1: 183: [28] income-tax
The income was first introduced in Britain in 1799.
Canto 1: 194: [29] Man's love is of man's life a thing apart/'Tis woman's whole existence"
These lines, among Byron's most often quoted, were probably borrowed from "Madame de Staël, De L'influence des passions, (1772).
Canto 1: 198: [30] Elle vou suit partout
"She follows you everywhere".
Canto 1: 198: [31] this story's actually true.
It is interesting to speculate to what extent the story is true. If Juan and Julia's bedroom debacle was part of Byron's personal experience, the tale seems not to have survived.
Canto 1: 212: [32] Non ego hoc ferrem calida juventâ / Consule Planco,"
"I would not have borne this in the heat of youth, when Plancus was consul" (Horace, Odes).
Canto 1: 222: [33] Go little book ... after many days.
This is the final stanza of Southey's "Epilogue to the Lay of the Laureate".
Canto 2: 12: [1] I can't but say ... our nautical existence.
This stanza is very poignant. Byron considered himself an involuntary social exile, and when he left England never intended to return.
Canto 2: 16: [2] Perhaps it may be lined with this my canto.
Trunk makers often used leaves from books to line their products. It's easy to imagine that badly-selling poetry might provide a ready source.
Canto 2: 26: [3] the wind increased at night/Until it blew a gale....
The details of the shipwreck in this and following stanzas were lifted by Byron from Shipwrecks and Disasters at Sea edited by Sir J.G. Dalyell and published at Edinburgh in 1812.
Canto 2: 56: [4] Battista ... (a name called shortly Tita)
A cameo reference to Byron's Venetian gondolier and servent to the end of his life, Giovanni Battista Falcieri.
Canto 2: 65: [5] Of any creditors the worst a Jew it is
Byron ran up a large debt to Jewish bankers during his days at Oxford. He was unable to repay them for many years.
Canto 2: 83: [6] if foes be food in hell
Refers to Book 33 of the Inferno, lines 76-78 (Longfellow translation):
When he had said this, with his eyes distorted,
The wretched skull resumed he with his teeth,
Which, as a dog's, upon the bone were strong.
'He' in this case being the Ugolino whom Byron names earlier in the stanza.
Canto 2: 84: [7] Truth is -- in a well.
"Truth lies at the bottom of a well" is proverbial. The idea has been traced back to Democritus.
Canto 2: 105: [8] pass'd the Hellespont, As ... Leander, Mr. Ekenhead, and I did.
The Hellespont, or Dardanelles, is the narrow straight through which the Black Sea issues into the Mediterranean. It was here that Leander drowned trying to reach his lover Hero. In 1810, Byron, accompanied by Lt. Ekenhead, swam it from Sestos on the European shore to Abydos in Asia. Byron, who, though crippled, was a strong swimmer since his University days, was proud of this feat and referred to it many times, often humorously. He wrote this poem in 1812.

Canto 2: 128: [9] He had an only daughter, call'd Haidée
Byron may have taken this name from Greek folk songs. According to Pratt, it means "a caress," or "the caressed one."
Canto 2: 130: [10] the good old man had no mush "nous"
For "nous" read the greek work spelled "nu,omicron,upsilon,sigma". It means "wit" or "sense". One of the great deficiencies of HTML at the time I am writing is that there is no convenient way to represent characters in non-Latin alphabets.
Canto 2: 137: [11] my grand-dad's "Narrative"
Captain John "Mad Jack" Byron wrote A Narrative of the Honourable John Byron (Commodore in a late expedition round the world), containing an account of the great distresses suffered by himself and his companions on the coast of Patagonia, from the year 1740, till their arrival in England, 1746; written by Himself.
Canto 2: 206: [12] Cæsar and Pompey, Mahomet, Belisarius
All men with wives who strayed or were rumoured to have done so:
Canto 3: 9: [1] of Death or of the Lady
"Death and the Lady" was a popular ballad first published in 1736. Marchand calls it "lugubrious", but you can decide for yourself:
Canto 3: 10: [2] But Dante's Beatrice and Milton's Eve / Were not drawn from their spouses, you conceive
Milton and Dante, like Shakespeare, Dryden, and indeed Byron, had troubled marriages. We may safely "conceive" that Haidée's character is not drawn from Lady Byron.
Canto 3: 29: [3] Dervises ... the Pyrrhic dance so martial
The dancing of the dervishes in Turkey was considered wild and orgiastic. The Greeks and Albanians had their own violent "war" dances in which they mimiced the taking and giving of blows.
Canto 3: 47: [4] good to govern -- almost as a Guelf
This is an oblique reference to the English royal house -- the House of Hanover. They descended from the German tribe of Guelf.

Byron was no admirer of the Hanoverian dynasty, but he was wise enough to avoid the criminal offense of directly abusing the Georges. Later poets were less careful. Winthrop Mackworth Praed, disguising his target somewhat, wrote "Epitaph on the Late King of the Sandwich Isles" on the death of George IV, a man known for his drunkenness and debauchery:

George III fared even less well with posterity, as this "clerihew" shows:

Walter Savage Lander provided an historical assessment of the Hanoverian line of kings:

Canto 3: 65: [5] The words which shook Belshazzar in his hall
Referring to Daniel's interpretation of the famous "writing on the wall" in Daniel chapter 5. This story was interesting to Byron, and he used it in several places. Perhaps the best known is his short 1815 poem, "The Vision of Belshazzar":

Canto 3: 79: [6] An Eastern anti-jacobin
The anti-jacobins were a party in England opposed to the principles and politics of the French Revolution. They urged adherence to the old tried-and-true English constitution and way of thinking.
Canto 3: 81: [7] "Vates irritabilis"
Literally, "irritable poet". Moore says it probably refers to the second chapter of Coleridge's Biographica Literaria, on the "supposed irritability of men of genius."
Canto 3: 85: [8] Pindar sang horse-races
Several of the Pindaric Odes refer to chariot races.
Canto 3: The Isles of Greece: [9]
"The Isles of Greece" is such powerful and concentrated verse, it seems a shame to clutter it with notes. Yet, there are many references that will mean little to modern readers. I will try to clear up a few of them here.

First note that this piece deals with the central passion of Byron's later life: the liberation of Greece from Turkey. It compares and contrasts the character of ancient Greeks with the modern and urges the latter to emulate their forbears. This is a theme that Byron treated at length in Canto II of Childe Harold.

Here are a few of the references that may be unclear:

Canto 3: 92: [10] Like Titus' youth, and Cæsar's earliest acts
The early lives of both Titus and Julius Cæsar were notable for profligacy according to Suetonius.
Canto 3: 93: [11] before his flighty pen/Let to the Morning Post
Southey wrote verses for the Tory Morning Post, for a guinea a week, from 1798 to 1803.
Canto 3: 93:[12] All are not moralists ... (milliners of Bath).
More abuse of Southey, Coleridge and Wordsworth. In 1794 the first two proposed the "Pantisocracy" plan to set up a utopian community on the Susquehanna. (The term "Pantisocrats" later was applied to the Lake Poets in general.) The earlier poems of Wordsworth (what Byron calls his "pedlar" poems) and of Coleridge (before he started selling them to Tory rags like the Morning Post) are generally considered their best. The "milliners of Bath" represent the Fricke sisters, Sarah (who married Coleridge) and Edith (who married Southey). Although they grew up gentile but poor in Bath, there is no evidence that they worked as milliners or dressmakers. According to Marchand, milliners then had the reputation of being of easy virtue.
Canto 3: 98:[13] He wishes for "a boat" to sail the deeps
See the opening lines of Peter Bell: 

    There's something in a flying horse,
    There's something in a huge balloon;
    But through the clouds I'll never float
    Until I have a little Boat,
    Shaped like the crescent-moon.
Canto 3: 99:[14] pray Medea for a single dragon
In the Medea of Euripedes, and probably in 18th-century exhbitions, Medea is depicted bearing the bodies of her children away in a cart pulled by dragons.
Canto 3: 101: [15] Ave Maria!
The Ave Maria passages in this and the next two stanzas probably were not read by contemporaries as further evidence of Byron's impiety. It was still fashionable to sneer at Catholic rites and beliefs. Nevertheless, Byron saw fit to insert a defense of his "devotion" in stanza 104: "My altars are the mountains and the oceans..."
Canto 4: 5: [1] Some have accused me of a strange design
It would be a very fine thing for someone to make digital versions of some of these bad reviews. Even facimile pages of such journals as the Eclectic Review would be welcome.
Canto 4: 7: [2] This is a liberal age, and thoughts are free
For just how illiberal Byron thought the age, see the preface.
Canto 4: 75: [3] Wounded and fetter'd, "cabin'd, cribb'd, confined,"
This is one of a half-dozen quotes from Macbeth in this Canto. The quotations are rather obscure, but they do make some passages of this Canto seem familiar even on first reading. Even in his letters, written far away from reference material, Byron shows an encyclopedic knowledge of Shakespeare. As Marchand points out, his references are usually not to well-known and often-quoted passages.
Canto 4: 84: [4] You was not last year at the fair of Lugo
The reason for Raucocanti's slide into collquial speech here is uncertain. Perhaps it is an upper-class affectation, like dropping the terminal "g" became in the 20th century. I think that Byron was trying to give the impression of two dandies chatting about opera, in stark contrast to their perilous situation.
Canto 4: 83: [5] "Arcades ambo," id est -- blackguards both.
Byron seems to have liked the phrase "Arcades ambo" (Arcadians both). He used it in a couple of letters. It's from Virgil's Bucolics.
Canto 4: 96: [6] "no one in hand can hold a fire" . . . not much less real.
The sense is that the mind cannot often suppress the senses, making Juan's numbness even more wonderful.
Canto 4: 98 : [7] the purer page/Of Smollet, Prior, Ariosto, Fielding
Byron frequently claimed that the morals of his readers, especially females, were far safer with him than with other writers. Here's a satiric passage on Thomas Little (an early pseudonym of Thomas Moore) from Byron's English Bards and Scotch Reviewers:

Canto 4: 103: [8] I canter by the spot each afternoon
In this and the following verses, Byron describes the vicinity of Ravenna where he was living when this Canto was written.
Canto 4: 108: [9] Oh ye who make the fortunes . . . tasting your Castalian tea!
The sense of this rather convoluted verse is that the poet mockingly begs the acceptance of the poem by polite society. If they do not accept it, he must "go to the ... cooks" who will presumably wrap their pies with the pages of his book. Ships wrecked on the Cornish coast were usually quickly plundered by the locals, and he 'fears' the same will happen to the poem. It is ironically that he compares Bluestocking tea with the inspiring waters of the spring of Castalia.
Canto 4: 109: [10] like Yorick's starling
Referring to a scene in Laurence Sterne's novel A Sentimantal Journey featuring the Rev. Mr. Yorick and a caged starling.
Canto 4: 110: [11] As someone somewhere sings
Southey in Madoc in Wales.
Canto 4: 110: [12] Blue as the garters . . . levee morn
A deep-blue garter worn at the left knee is a symbol of the Order of the Garter, the highest English order of chivalry. It was of course a rich man's club. Members might well expect to be invited to a "levee morn" or formal morning reception in the Regent's bed-chamber.
Canto 4: 111: [13] The loveliest, chastest, best, but -- quite a fool.
Perhaps a reference to Lady Charlemont, who I believe was the wife of the Irish 2d Earl. In his journal for December 1, 1813, Byron refers to a possible evening with her:

Perhaps that blue-winged Kashmirian butterfly of book-learning, Lady Charlemont, will be there. I hope so; it is a pleasure to look upon that most beautiful of faces.

In a November entry he refers to "all the Blues, with Lady Charlemont at their head".

Canto 4: 112: [14] An airy instrument . . . to ascertain the atmospheric state
The instrument, called a cyanometer was actually invented by Saussure. The idea was that the density of the atmosphere is correlated with the shade of blue of the sky. The instrument worked, but not well and not at all at night or under clouds.