Michelangelo had hitherto acquired no practical acquaintance with the art of fortification. That the system of defence by bastions was an Italian invention (although Albert Dürer first reduced it to written theory in his book of 1527, suggesting improvements which led up to Vauban's method) is a fact acknowledged by military historians. But it does not appear that Michelangelo did more than carry out defensive operations in the manner familiar to his predecessors. Indeed, we shall see that some critics found reason to blame him for want of science in the construction of his outworks. When, therefore, a difference arose between the controller-general of defences and the Gonfalonier upon this question of strengthening S. Miniato, it was natural that the War Office should have thought it prudent to send their chief officer to the greatest authority upon fortification then alive in Italy. This was the Duke of Ferrara. Busini must serve as our text in the first instance upon this point. "Michelangelo says that, when neither Niccolò Capponi nor Baldassare Carducci would agree to the outworks at S. Miniato, he convinced all the leading men except Niccolò of their necessity, showing that Florence could not hold out a single day without them. Accordingly he began to throw up bastions with fascines of tow; but the result was far from perfect, as he himself confessed. Upon this, the Ten resolved to send him to Ferrara to inspect that renowned work of defence. Thither accordingly he went; nevertheless, he believes that Niccolò did this in order to get him out of the way, and to prevent the construction of the bastion. In proof thereof he adduces the fact that, upon his return, he found the whole work interrupted." Furnished with letters to the Duke, and with special missives from the Signory and the Ten to their envoy, Galeotto Giugni, Michelangelo left Florence for Ferrara after the 28th of July, and reached it on the and of August. He refused, as Giugni writes with some regret, to abandon his inn, but was personally conducted with great honour by the Duke all round the walls and fortresses of Ferrara. On what day he quitted that city, and whither he went immediately after his departure, is uncertain. The Ten wrote to Giugni on the 8th of August, saying that his presence was urgently required at Florence, since the work of fortification was going on apace, "a multitude of men being employed, and no respect being paid to feast-days and holidays." It would also seem that, toward the close of the month, he was expected at Arezzo, in order to survey and make suggestions on the defences of the city.