Leonardo certainly was in relation with some persons belonging to that circle when he published in 1220 another more extensive work, De practica geometriae, which he dedicated to the imperial astronomer Dominicus Hispanus.
The other works consist of the Practica geometriae and some most striking papers of the greatest scientific importance, amongst which the Liber quadratorum may be specially signalized.
In his Practica geometriae plain traces of the use of the Roman agrimensores are met with; in his Liber abaci old Egyptian problems reveal their origin by the reappearance of the very numbers in which the problem is given, though one cannot guess through what channel they came to Leonardo's knowledge.
The second work of Leonardo, his Practica geometriae (1220) requires readers already acquainted with Euclid's planimetry, who are able to follow rigorous demonstrations and feel the necessity for them.
Among the writers it may be sufficient to mention here Gariopontus; Copho, who wrote the Anatome porci, a well-known medieval book; Joannes Platearius, first of a family of physicians bearing the same name, whose Practica, or medical compendium, was afterwards several times printed; and Trotula, believed to be the wife of the last-named.
A more important work, the Practica seu lilium medicinae, of Bernard Gordon, a Scottish professor at Montpellier (written in the year 1307), was more widely spread, being translated into French and Hebrew, and printed in several editions.
Of these two physicians the first probably, the latter certainly, was educated and practised abroad, but John Gaddesden (1280?-1361), the author of Rosa anglica seu Practica medicinae (between 1305 and 1317), was a graduate in medicine of Merton College, Oxford, and court physician.
Janus Cornarius, from whom this is quoted, laments, however, that the Arabians still reigned in most of the schools of medicine, and that the Italian and French authors of works called Practica were still in high repute.
His principal works are contained in four volumes comprised under the general title Filosofia dello spirito: (1) Estetica come scienza dell' espressione e linguistica generale, (2) Logica come scienza del concetto puro, (3) Filosofia della practica: economia ed etica and (4) Teoria e storia della storiografia.
It is entitled Practica D.
He wrote also De Geometria speculativa (Paris, 1530); De Arithmetica practica (Paris, 1502); De Proportionibus (Paris, 1495; Venice, 1505); De Quadratura Circuli (Paris, 1 495); and an Ars Memorativa, Sloane MSS.
This article relates to the Western Cathars, as they appear (1) in the Cathar Ritual written in Provencal and preserved in a 13th -century MS. in Lyons, published by Cledat, Paris, 1888; (2) in Bernard Gui's Practica inquisitionis haereticae pravitatis, edited by Canon C. Douais, Paris, 1886; and (3) in the proces verbal of the inquisitors' reports.
C. Conybeare, The Key of Truth (Oxford, 1898); Henry C. Lea, History of the Inquisition (New York, 1888); C. Douais, L' Inquisition (Paris, 1906), and his Les Heretiques du midi au XIII e siècle (Paris, 1891); Les Albigeois (Paris, 1879); also Practica Inquisitionis (of Bernard Gui or Guidon), (Paris, 1886); L.
They next appear to have been used by Daniel Schwenter (1585-1636) in a Geometrica Practica published in 1618.