This well-earned celebrity may have been owing in no small degree to a mathematical and astronomical work produced by the philosopher after his return to court. Sacrobosco, the famous English astronomer, had just risen into notice by his treatise on the _Sphere_. This book was not indeed very remarkable in itself, but it obtained an extraordinary currency during the Middle Ages, and after the invention of printing as well as before it:[208] a popularity chiefly due, we may believe, to its suggestiveness, which caused many of the learned to enrich the _Sphere_ of Sacrobosco with their own notes and observations. One of the first to do so was Michael Scot. His commentary on the work of Holywood contains several subtle inquiries and determinations regarding the source of heat, the sphericity of the heavenly bodies, and other matters, which have been repeated by Libri with the remark that their author must have been far in advance of his times.[209] We may notice here a curious legend of Naples to which Sir Walter Scott has drawn attention in the account he gives of his great namesake.[210] It would seem to suggest that this age, perhaps by means of Michael Scot, was acquainted with philosophical instruments rarer if not more useful than the astrolabe. The romance of _Vergilius_ tells how that hero founded ‘in the middes of the see a fayer towne, with great landes belongynge to it; … and called it Napells. And the fandacyon of it was of egges, and in that towne of Napells he made a tower with iiii corners, and in the toppe he set an apell upon an yron yarde, and no man culd pull away that apell without he brake it; and thoroughe that yren set he a bolte, and in that bolte set he a egge. And he henge the apell by the stauke upon a cheyne, and so hangeth it still. And when the egge styrreth, so shoulde the towne of Napells quake; and when the egge brake, then shulde the towne sinke,’ The reference here is of course to the _Castel del Ovo_ at Naples, a fortress which we know to have been built, or at least strengthened, by Frederick II. What if the rest of the legend embalm, like a fly in amber, the tradition, strangely altered, of some instrument set up there to measure the force of the earthquakes so prevalent in that part of Italy?