(_e_) He was the first to pivot a magnetized needle and surround it with a graduated circle, Figs. 2 and 3.[2] (_f_) He determined the position of an object by its magnetic bearing as done to-day in compass surveying; and (_g_) He introduced into his perpetual motion machine, Fig. 4, the idea of a magnetic motor, a clever idea, indeed, for a thirteenth century engineer. This rapid summary will serve to show that the letter of Peregrinus is one of great interest in physics as well as in navigation and geodesy. For nearly three centuries, it lay unnoticed among the libraries of Europe, but it did not escape Gilbert, who makes frequent mention of it in his _De Magnete_, 1600; nor the illustrious Jesuit writers, Cabæus, who refers to it in his _Philosophia Magnetica_, 1629, and Kircher, who quotes from it in his _De Arte Magnetica_, 1641; it was well known to Jean Taisnier, the Belgian plagiarist, who transferred a great part of it verbatim to the pages of his _De Natura Magnetis_, 1562, without a word of acknowledgment. By this piece of fraud, Taisnier acquired considerable celebrity, a fact that goes to show the meritorious character of the work which he unscrupulously copied. This memorable letter is divided into two parts: the first contains ten chapters on the general properties of the lodestone; the second has but three chapters, and shows how the author proposed to use a lodestone for the purpose of producing continuous rotation. There are many manuscript copies of the letter in European libraries: the Bodleian has six; the Vatican, two; Trinity College, Dublin, one; the Bibliothèque Nationale, Paris, one; Leyden, Geneva and Turin, one each. The Leyden MS. has acquired special notoriety from a passage which appears near the end of it in which reference is made to magnetic declination and its value given: but Prof. W. Wenckebach, of The Hague, has shown[3] that the lines are spurious, having been interpolated in the manuscript in the early part of the sixteenth century. The Leyden manuscript has also led some writers to believe in a fictitious author of the letter, one Peter Adsiger, or Petrus Adsigerus. As said above, Sigerus was the name of his countryman, to whom Peregrinus addressed his letter, the _Epistola ad Sigerum_, from the trenches at Lucera, in August, 1269. Magnetic declination was unknown to Peregrinus, else he would not have written the following words: “Wherever a man may be, he finds the lodestone pointing to the heavens in accordance with the position of the meridian” (Chapter X). Of course, the geographical meridian is the one here meant, as the necessity of a distinct magnetic meridian had not yet occurred to any one.