took turns in leading, so that each in turn might come first to the camp.
§ 143. Rivers were crossed either by fords or by bridges. The Romans could cross deeper fords than we, as they had no powder to keep dry. Cæsar preferred fords whenever practicable, as they required no previous preparation. Some times an artificial ford was made. Often, when the current was strong, a line of cavalry was stationed up stream from the point of crossing, and another line down stream, and the infantry crossed in this shelter. The upper line of cavalry broke the force of the current, and the lower line saved any men who were carried from their footing.
B. G. VII, 56.
§ 144. When fords were not available, bridges had to be built. These were of many kinds. The simplest were to cross a mere ravine, and consisted of long tree trunks covered with branches and earth. The most elaborate of which we know was the footway 40 ft. wide with which Cæsar twice spanned the Rhine. A river in Spain he bridged by sinking baskets filled with stones, as foundations for his piers. Other streams were crossed by bridges of boats. A bridge of any importance had to be protected by strong fortifications at each end ; and, when it was desired to retain it, these were held by suitable garrisons (praesidia).
Cæsar's bridges on the Rhine (Fig. 26) were of this description. They were masterpieces of military engineering, and were held securely while the army moved into Germany.
B. G. IV, 17; VI, 9.
§ 145. The Romans distinguished two kinds of camp: the field, or summer camp (castra aestiva), made at the close of each day's march, to be abandoned the next morn