Happily, he was spared the horrors of a second siege—that of Paris in the hands of the Commune. The first had left poignant memories never to be effaced. Yet his vocation was so firmly fixed that he was not to be turned from it even by this discouraging commencement. The possibility of avenging national humiliation braced his energies and kept him continually at work preparing for the inevitable day through more than two score years. He became one of the most authoritative teachers of the army. At St. Cyr, which he had attended as student, he became professor, and when Fontainebleau started its artillery course, it was he who directed it for the benefit of subalterns. Whilst he was professing artillery at St. Cyr, a great controversy raged on the subject. It was clear that France had been out-classed by Germany in field-guns; it was one of the causes of her defeat. The guns were more powerful, more accurately aimed, and more quickly served. The Germans had learnt the art of shooting, which the French had neglected. Though the De Reffye cannon was much better than its predecessor—a muzzle-loader, firing an eight-pound shell, which broke only at two distances—it was still far behind the German arm. The young war professor pronounced strongly in favour of power. The field-gun, above all, must be all-powerful, he said; mobility could come afterwards. The rival camp protested that mobility should come first; you could mass your light and handy guns and obtain the power required. But Maunoury unveiled what he considered to be the sophistry of this argument. The result of all this heat was the 75 mm., a model field-gun, which wrought wonders for France in the Great War. But its efficiency was not unconnected with an excellent shell.