Davidis Arnoldi Conradi Cryptographia Denudata, sive ars deciferandi quae occulte scripta sunt in quocunque linguarum genere, praecipue in Germanica, Batava, Latina, Anglica, Gallica, Italica, Graeca [Cryptograph Stripped Bare, or the art of deciphering what secrets are written in any kind of languages, in particular in German, Dutch, Latin, English, French, Italian, and Greek] (Leyden, 1739) (Google)
This work in Latin explains general theory of deciphering by frequency analysis and provides specific nature of various languages in "rules" and "propositions". According to the dedication, the author (David Arnold Conradus [Conrad]) had a brother (Justus Anthony Conradus) who was a secretary of the chamber ["camera"] of the King of Britain.
The work consists of the following sections:
Introduction ... p.5
Section I. General Theory (Proposition I to XVI; Rule 1 to 9)... p.15
Section II. Particular Theory
Chapter I. German Language (Proposition XVII to XXXIV; Rule 10; Problem 1 to 2) ... p.20
Chapter II. Dutch Language (Proposition XXXV to LII; Rule LII; Problem 1 to 2) ... p.27
Chapter III. Latin Language (Proposition LIII to LXVI; Problem 1 to 2) ... p.35
Chapter IV. English Language (Proposition LXVII to LXXIX; Problem 1 to 2) ... p.40
Chapter V. French Language (Proposition LXXX to LXXXVII; Problem 1 to 2) ... p.47
Chapter VI. Italian Language (Proposition LXXXVIII to CIII; Problem 1-2) ... p.52
Chapter VII. Greek Language (Proposition CIV to CXIX; Problem 1-2) ... p.60
Addenda (Key to the Problems) ... p.69
Errata ... p.73
Catalog of Books by the Publisher (Philippus Bonk)
An English version of the work was printed in Gentleman's Magazine (1742) (Google) under the title "Cryptographia Denudata. The Art of Deciphering deduced from Principles, and explained by Examples in the German, Dutch, Latin, English, French, Italian, and Greek Languages." The beginning sentence "The Art of Deciphering being an abstruse subject, on which I know not that any Treatise has ever been professedly written, ..." is given a footnote "A Treatise was written on this Subject by B. Porta; and a Collection of Letters written in Cipher, and deciphered by Dr. Wallis, are now in the Library of Oxford." The arbitrary symbols used in the problems in the Latin edition were replaced by other symbols of which type could be readily found.
The Introduction, General Theory, and the chapter of German Language are on p.133-135; the chapters of Dutch, Latin are on p.185-186; the chapters of English and French are on p.241-242; the chapters of Italian and Greek are on p.473-475.
In the Peninsular Campaign, a British officer, George Scovell, appointed to superintend all the communications of the army in the summer of 1811, deciphered many intercepted French letters in code/cipher (see another article). He was given a small notebook containing a handwritten copy of this work by Conradus (Urban (2001), The Man Who Broke Napoleon's Codes p.102-104). Although the teaching of Conradus could only be useful to the simplest kind of cipher which the British encountered at the beginning, Scovell appears to have cherished the notebook and wrote in it his annotations. (Urban (2001) p.116-117; 125, 177, 312; 189) The notebook is now preserved as WO37/9 (UK National Archives)
It is noted that there are some differences from the version of Gentleman's Magazine. In particular, "Rule 3" of the notebook reads "Writings of any length are most easy to decypher from the frequent recurrence and combination of the same letters." Thus, it would have been independently translated from the original Latin edition.
It is remarkable that the British army brought in its expedition to the Peninsula this work published more than half a century before. Indeed, there was not much publication in this field at the time. Moreover, the databook style this work, enumerating characteristics of several languages, must have been considered handy in the field.