The horsemen sped away like the bearers of the fiery cross. Caesar learnt from deserters that they had gone out, and understood the message which they carried. Already he was besieging an army far outnumbering his own. If he persevered, he knew that he might count with certainty on being attacked by a second army immeasurably larger. But the time allowed for the collection of so many men might serve also to prepare for their reception. Vercingetorix said rightly that the Romans won their victories, not by superior courage, but by superior science. The same power of measuring the exact facts of the situation which determined Caesar to raise the siege of Gergovia decided him to hold on at Alesia. He knew exactly, to begin with, how long Vercingetorix could hold out. It was easy for him to collect provisions within his lines which would feed his own army a few days longer. Fortifications the same in kind as those which prevented the besieged from breaking out would serve equally to keep the assailants off. His plan was to make a second line of works--an exterior line as well as an interior line; and as the extent to be defended would thus be doubled, he made them of a peculiar construction, to enable one man to do the work of two. There is no occasion to describe the rows of ditches, dry and wet; the staked pitfalls; the cervi, pronged instruments like the branching horns of a stag; the stimuli, barbed spikes treacherously concealed to impale the unwary and hold him fast when caught, with which the ground was sown in irregular rows; the vallus and the lorica, and all the varied contrivances of Roman engineering genius. Military students will read the particulars for themselves in Caesar's own language. Enough that the work was done within the time, with the legions in perfect good humor, and giving jesting names to the new instruments of torture as Caesar invented them. Vercingetorix now and then burst out on the working parties, but produced no effect. They knew what they were to expect when the thirty days were out; but they knew their commander, and had absolute confidence in his judgment.