Following an extensive tour of Great Britain and Ireland lasting more than four years, Lhuyd began work on Glossography, the first volume of a planned four-volume set, Archæologia Britannica, which combined innovative methods of historical linguistics, language comparison, and field research, to establish a genetic relationship between the Welsh, Cornish, Breton, Irish, Scottish Gaelic, and Gaulish languages.

[1] His methodology allows a systematic study of etymology, including a focus on regular sound changes, equivalence or similarity of meaning of cognates, and shared morphology, and emphasises that the basis of comparison should be the most basic parts of a language's core vocabulary.

[10] In 1693, Edward Lhuyd, an antiquarian, naturalist, botanist, geographer, and philologist, and recently appointed Keeper of the Ashmolean Museum at the University of Oxford,[11] was invited to contribute to the Welsh sections of William Camden's Brittania, a survey of Great Britain and Ireland.

[11] His work on this revision motivated him to begin his own magnum opus, Archæologia Britannica, an envisaged comparative study of the shared characteristics of the languages, archaeology, and culture of Wales, Cornwall, Brittany, Scotland, and Ireland.

[12] Following the publication of Britannia in 1695, Lhuyd published A Design of a British Dictionary, Historical & Geographical: with an Essay entitl'd Archæologia Britannica, seeking subscribers to fund the research, fieldwork, and eventual publication of what was initially conceived as a multi-volume work, Archæologia Britannica or An Account of the Ancient Languages, Customs, and Monuments of the British Isles.

[11] Lhuyd and his companions travelled Britain and Ireland for four years, studying and collecting manuscripts, ancient artefacts, and fossils, describing architecture and monuments, and recording local culture and spoken languages.

[17] Having trained others to take on his duties at Oxford and collected some funding, Lhuyd, along with his three assistants, began his tour in May 1697, travelling through Gloucestershire and the Forest of Dean, and reaching Chepstow on 13 May.

[19] On account of their research activities during their travels in Wales, Lhuyd and his assistants were suspected of being Jacobite spies, conjurers, or tax collectors by suspicious locals.

"[22] From Wales, Lhuyd and his team reached Ireland in July or August 1699, landing in Dublin, then travelled to Antrim and visited Newgrange and the Giant's Causeway.

They then took the ferry to Scotland in late September or October, before returning to northern Ireland by boat in January 1700, visiting Ulster, Connaught, and Munster, arriving in Killarney in July.

[19] In Cornwall, Lhuyd was able to gather information about the Cornish language by listening to native speakers, especially the parish of St. Just, from local antiquarians such as Nicholas Boson and John Keigwin, and from three manuscripts he was able to study, Pascon agan Arluth, the Ordinalia, and Gwreans an Bys.

[27] Lhuyd's team also produced sketches and plans of antiquities and ancient monuments, including Boskednan stone circle and Chûn Castle.

[31] John Keigwin's reaction to Lhuyd's arrival in Mousehole is satirized in Alan Kent's Anglo-Cornish dialect play, Dreaming in Cornish:[32] Still, on his head he wore his periwig, even though it looked more like a geat gannet's nest than the attire of a gennelman.

After authorities found nothing treasonous in the seized documents, they were released, but then forced to leave the kingdom, as war "was already declar'd against the Empire, the Dutch, and the English.

It was not published until 1707, however, due to a lack of suitable movable type for the complex orthography used in the volume, which consisted of an extended Latin alphabet combined with a variety of diacritics, meaning only one compositor could perform the task.

He expresses his hope that the book will provide a clearer understanding of the ancient languages of Britain and Ireland, and thanks Mansel for his generosity and promotion of scholarship in general.

[46] Lhuyd then apologises for the time it has taken to produce the first volume, stating that he did not initially intend to travel for so long or in so much detail, or to write such a large essay.

[47] Lhuyd outlines his migration model for the Celtic settlement of Britain:[6][48] Having now related what none have hitherto made mention of, namely, first that the old inhabitants of Ireland consisted of two nations, Gwydhelians and Scots.

And fourthly that the said Gwydhelians of England and Wales were the inhabitants of Gaul before they came into this island.This title examines lexical and phonological correspondences in different languages, as well as semantic changes.

What I aim'd at therein, was the shewing by a collection of examples methodized, that etymology is not, as a great many, till they have considered it with some application, are apt to be perswaded, a speculation merely groundless or conjectural.

It may be of some use to the Curious in Etymology, if we distinguish Permutation of Letters into three sorts; which may be call'd Classical, Idiomatal, and Accidental.In "Observation XVIII", Lhuyd goes into some detail describing vowel variations in cognate words in the Celtic languages.

[63] "Observation XIX", focusing on the labial consonants /p/, /b/, /f/, /v/, and /m/, describes lenition, part of the Celtic grammatical mutation system (for example Welsh pen 'head' becoming i ben 'his head').

[68] In "Observation XXIV", Lhuyd describes how compounds or phrases may be translated from one language to another, or: "Words Deriv'd From One Common Origin as to Signification Tho' of No Affinity in Sound."

He gives examples including Welsh gloin dẏu 'butterfly', equivalent to Cornish tikki deu, and Scottish Gaelic dealan de.

[82] Lhuyd then provides a discussion of the ancient manuscripts he is aware of, along with a synopsis of the orthographic variations and his interpretation of the relationship of the written word to the pronunciation in these documents.

[85] There is a further guide to reading ancient manuscripts, in which Lhuyd discusses how particles with grammatical function are often joined to other words in old Welsh and Cornish documents.

[69] Written by David Parry, one of Lhuyd's assistants, this title features a section with English headwords, followed by a wordlist of Latin lemmata, glossed with basic vocabulary from various European languages.

[110] George Hickes, in a letter to Lhuyd, wrote that "so I doubt not but it will be very satisfactory to all men, who have a genius for antiquity, and the more learned and judicious they are, the more they will approve it, and be pleased with it.

[113] According to the Evans and Roberts edition, Glossography "gave etymology a rational basis in the conceptual framework of the seventeenth-century scientific thought and thereby set the comparative method on firmer ground".

[119] Walsham states that the included Cornish grammar and vocabulary "helped to lay foundations for the initiatives of Thomas Tonkin and Richard Polwhele and Jenner's revival efforts.