There was some difference of opinion in respect to the hour of departure. The daytime, it was argued by some, would be preferable, since it would enable them to see the nature and extent of their danger, and to provide against it. Darkness would be much more likely to embarrass their own movements than those of the enemy, who were familiar with the ground. A thousand impediments would occur in the night, which might prevent them acting in concert, or obeying, or even ascertaining, the orders of the commander. But, on the other hand, it was urged that the night presented many obvious advantages in dealing with a foe who rarely carried his hostilities beyond the day. The late active operations of the Spaniards had thrown the Mexicans off their guard, and it was improbable they would anticipate so speedy a departure of their enemies. With celerity and caution, they might succeed, therefore, in making their escape from the town, possibly over the causeway, before their retreat should be discovered; and, could they once get beyond that pass of peril, they felt little apprehension for the rest. The general had already superintended the construction of a portable bridge to be laid over the open canals in the causeway. This was given in charge to an officer named Magarino, with forty soldiers under his orders, all pledged to defend the bridge to the last extremity. The bridge was to be taken up when the entire army had crossed one of the breaches, and transported to the next. There were three of these openings in the causeway, and most fortunate would it have been for the expedition, if the foresight of the commander had provided the same number of bridges. But the labor would have been great, and the time was short. At midnight the troops were under arms, in readiness for the march. Mass was performed by Father Olmedo, who invoked the protection of the Almighty through the awful perils of the night. The gates were thrown open, and, on the first of July, 1520, the Spaniards for the last time sallied forth from the walls of the ancient fortress, the scene of so much suffering and such indomitable courage.