I admit that I must use part of Marshal DacDonald’s Recollection, even if the Marshal admits the he did not intend to write a menoir, but merely his recollections of events which transpired long ago.
From his reminiscences of the event, he informs us that he was in a desperate position, which required a necessary risk. With that said, I fully understand that he is recalliing the event.
The French system of 1805 was established upon l'ordre perpendiculaire or perpendicular order, which acknowledged the general principles of the Reglement de 1791, but it must be remembered that the Reglement was not a settlement of older instructions of discipline. But they were in fact developed after the Seven Years War, under the influence of the Prussian methods and their military system. It was therefore a continuation of the Prussian tactics in all its forms, and French sought to adapt their own system of movements that best suited their military system as years progressed.
French Tacticians at the time believed that all troops should possess; 1st, strength; 2nd activity, which produces celerity; and 3rd, universal mobility: by this latter quality must be understood as a formation adapted to all kinds of ground, and to be able to act against all species of assailing bodies of troops; also, that the body of troops use their arms to the greatest effect against the enemy.
With this in mind; it came about that a compromise of both the advantages of the light order en mousqueterie, and order by depth (light order not to exceed six ranks, but not less than two ranks. While in the old linear system; it relied upon an order of depth which did not exceed 12 to 15 ranks.
Nevertheless; according to the leading Tacticians of the time; there were those who wanted to combine the two depths into one system.
Therefore getting back to Marshal MacDonald, he relates how he was preparing to assault the Austrian position along the Russbach, when he received orders
from the Emperor to redirect his advance.
It was also understood that there was a gap in the French line toward which a great body of Austrian cavalry were headed.
with that said, it was common practice as a measure against cavalry when infantry were deployed to form lines doubled, such as the British were found in four ranks at Waterloo. -Lines doubled were more common that thought by modern historians.
From 1797 onward it was common to find a body of troops en colonnes serrées et de la ligne déployée (flanking battalions in column and center battalions doubled while deployed).
With a great body of troops, it was possible to find them in a formation called a “colonne vuide”, this was a configuration of troops from battalion to multi-division level arranged to combat cavalry whilst manoeuvring towards an enemy's line of battle. This same formation was used in the 1700s, but as a defensive evolution for retiring before the enemy.
The front of the colonne vuide was composed of battalions in line doubled. The flanks of the colonne vuide were composed of battalions in column of divisions, which replaced the colonne d’attaque. At the rear of the colonne vuide, it would be protected by a great body of cavalry.
Though the colonne vuide had been around since the 1700’s, it was given new life within l’ordre perpendiculair. Marshal MacDonald decide to use a compound evolution that was within the general principles of the French military system for grandes manoeuvres. For those who critic the formation used by marshal MacDonald, it is plainly understood that they themselves do not understand the system used by the French.
Marshal MacDonald made a sound judgement when he formed his great body of troops in a manoeuvre de principles (military term for a great body of troop that manoeuvre like a single battalion) while using the colonne vuide.