But the career of Vauban forms the most marked and prominent era in the history of fortification; it constitutes the connecting link between the rude sketches of the earlier engineers, and the well-established form which the art has since assumed. In his earlier works we find many of the errors of his predecessors; but a gradual change seems to have been wrought in his mind by reflection and experience, and these faults were soon remedied and a new and distinct system developed. Vauban has left no treatise upon his favorite art, and his ideas upon fortification have been deduced from his constructions, and from detached memoirs left among his papers. The nature of his labors, and the extent of his activity and industry, may be imagined from the fact that he fought one hundred and forty battles, conducted fifty-eight sieges, and built or repaired three hundred fortifications. His memoirs, found among his manuscript papers, on various military and political subjects, are numerous, and highly praised even at the present day. But his beautiful and numerous constructions, both of a civil and military character, are real monuments to his genius. The best illustrations of his principles of fortification occur at Lille, Strasbourg, Landau, Givet, and Neuf-Brisack. His writings on mines, and the attack and defence of places, are, by the profession, regarded as classic. His improvements in the existing method of attack gave great superiority to the arms of his countrymen, and even enabled him to besiege and capture his rival Coehorn, in his own works. He died in 1707, and was soon succeeded by Cormontaigne. The latter did not attempt the introduction of any new system, but limited himself to improving and perfecting the plans of his illustrious predecessors. His improvements, however, were both extensive and judicious, and are sufficient to entitle him to the place he holds as one of the ablest military engineers the world has ever produced. His works on the subject of fortification, besides being elegantly written, contain the most valuable information of any works we have. His most admired constructions are to be found at Metz, Thionville, and Bitche. The beautiful crown works of Billecroix, at Metz, are perfect models of their kind. Cormontaigne died in 1750. Cotemporary with him were Sturin and Glasser. The former deviated but slightly from the systems of his predecessors, but the latter invented several ingenious improvements which gave him great reputation. Next follows Rosard, a Bavarian engineer; and Frederick Augustus, king of Poland, who devoted himself particularly to this art. The former casemated only the flanks of his works, but the latter introduced casemate fire more extensively than any one who had preceded him. In France, Belidor and De Filey published about the middle of the last century. They were both able engineers but their systems were inferior to that of Cormontaigne.