Chap. III. He constructs “a wheel which shall be constantly in motion,” by making a very thin concave, silver case, after the manner of a mirror, suitably perforated, around the rim of which he inserts small iron nails, or teeth, bent closely toward each other and which he then places upon an immovable axis so that it may revolve easily.” He continues: “Let a silver wire be added to this axis, fixed to it and placed between two bowls on the end of which let a magnet be set, prepared in this manner. Let it be rounded and its poles ascertained, as before indicated; afterwards, let it be fashioned in the shape of an egg with the poles intact, and let it be somewhat filed down in two intermediate and opposite parts with the object of its being compressed and occupying less space so that it may not touch the inner walls ... let the magnet be placed on the wire ... and let the North pole be somewhat inclined towards the small teeth of the wheel so that it may exercise its power ... so that each tooth shall arrive at the North pole and, owing to the impetus of the wheel, shall pass it by and approach the Southern quarter. Thus every small tooth will be in a perpetual state of attraction and avoidance. And, in order that the wheel may perform its duty with greater rapidity, insert, between the cases, a small round brass or silver pebble of such size that it may be caught between any two of the small teeth, so that, as one part of the wheel comes uppermost, the pebble may fall to the opposite part. Wherefore, whilst the motion of the wheel is perpetual on one side, the same will be in the case of the pebble on the other side, or the fall of the pebble caught between any two of the teeth will be perpetual to the opposite side because as it is drawn towards the centre of the earth by its weight, it assists the motion by not suffering the small teeth to remain at rest in front of the stone. Let there be spaces, however, between the small teeth conveniently curved, so as to catch the pebble as it falls in the way the present description indicates.” [Illustration: Petrus Peregrinus. Facsimile of a Ms. at the Bodleian Library, of the “Epistola de Magnete,” wherein is described the earliest known pivoted compass.] Gilbert alludes to this perpetual-motion engine as having been devised or delineated by Peregrinus after he had got the idea from others (“De Magnete,” Book II. chap. xxxv.), and says that Jerome Cardan writes (“Opera,” Batav., 1663; “De Rerum Varietate,” Book, IX. chap. xlviii.) he could construct one out of iron and loadstone--not that he ever saw such a machine; that he merely offers the idea as an opinion and quotes from a report of the physician Antonio de Fantis of Treviso published in “Tabula generalis ac mare magnum scotice subtilitatis....”