The letter was addressed from the trenches at Lucera, Southern Italy, in August, 1269, to Sigerus de Foucaucourt, his “amicorum intimus,” the dearest of friends. A more enlightened friend, however, than the knight of Foucaucourt was Roger Bacon, who held Peregrinus in the very highest esteem, as the following glowing testimony shows: “There are but two perfect mathematicians,” wrote the English monk, “John of London and Petrus de Maharne-Curia, a Picard.” Further on in his _Opus Tertium_, Bacon thus appraises the merits of the Picard: “I know of only one person who deserves praise for his work in experimental philosophy, for he does not care for the discourses of men and their wordy warfare, but quietly and diligently pursues the works of wisdom. Therefore, what others grope after blindly, as bats in the evening twilight, this man contemplates in all their brilliancy because he is a master of experiment. Hence, he knows all natural science whether pertaining to medicine and alchemy, or to matters celestial and terrestrial. He has worked diligently in the smelting of ores as also in the working of minerals; he is thoroughly acquainted with all sorts of arms and implements used in military service and in hunting, besides which he is skilled in agriculture and in the measurement of lands. It is impossible to write a useful or correct treatise in experimental philosophy without mentioning this man’s name. Moreover, he pursues knowledge for its own sake; for if he wished to obtain royal favor, he could easily find sovereigns who would honor and enrich him.” This last statement is worthy of the best utterances of the twentieth century. Say what they will, the most ardent pleaders of our day for original work and laboratory methods cannot surpass the Franciscan monk of the thirteenth century in his denunciation of mere book learning or in his advocacy of experiment and research, while in Peregrinus, the mediævalist, they have Bacon’s impersonation of what a student of science ought to be. Peregrinus was a hard worker, nor a mere theorizer, preferring, Procrustean-like, to make theory fit the facts rather than facts the theory; he was a brilliant discoverer who knew at the same time how to use his discoveries for the benefit of mankind; he was a pioneer of science and a leader in the progress of the world. An analysis of the “Epistola” shows that (_a_) Peregrinus was the first to assign a definite position to the poles of a lodestone, and to give directions for determining which is north and which south; (_b_) He proved that unlike poles attract each other, and that similar ones repel; (_c_) He established by experiment that every fragment of a lodestone, however small, is a complete magnet, thus anticipating one of our fundamental laboratory illustrations of the molecular theory; (_d_) He recognized that a pole of a magnet may neutralize a weaker one of the same name, and even reverse its polarity;