=281. Barrier Fortresses; System of Encampments; Flexible Battle Tactics; Siege Warfare.=--Into the details of the Roman war machine we cannot enter. We cannot discuss the wonderful system of barrier fortresses along the junction of the Rhine and Danube upon which the northern tribes beat in vain, nor the newly completed “Wall of Hadrian” sundering peaceful and guarded Britain from the stark savagery of Caledonia. We cannot explain the scientific system of temporary encampments, whereby every night--when a legion is on the march,--it occupies a square of ground fortified by solid palisades and with every tent in precisely the same spot as in the old camp of the preceding night--a method insuring that every camp becomes practically a fortress, almost impregnable in case of a defeat in the field. We cannot visit the permanent garrison towns, such as Colonia Agrippina (Cologne) on the Rhine, or Vindobona (Vienna) on the Danube, where extensive cities, with all the paraphernalia of civilization, have grown up around the cantonments on the very edge of raw barbarism. It is still less possible to offer here a discussion of the flexible legionary battle tactics, whereby each particular foe is met with the formations most formidable to his special arms and weaknesses; and of the carefully adjusted order of march whereby an army can move with all its baggage train through a hostile country defiant of any ordinary harassment and flank attack. We must pass over also the system of siege warfare, and the use of long-range casting engines--a genuine artillery; and finally the wonderfully scientific engineering service, building high-roads through deserts, and throwing strong bridges even across such mighty streams as the Rhine, and--on Trajan’s Dacian campaigns--the Danube. [Illustration: STORMING A BESIEGED CITY: casting engines in foreground.] =282. Limited Size of the Imperial Army: its Great Efficiency.=--Two or three things about the army, however, call for particular comment. The size of these forces seems decidedly small, considering the vast extent of the Empire, the slow communications, the careful demilitarizing of the provincials, and the absence of any reserve corps or efficient militia. The thirty legions (5000 to 6000 men each) reckon perhaps 175,000 troops of the line. The Prætorians at Rome, the heterogeneous and scattered auxiliary cohorts, the small naval force, and other armed groups at the command of the government, in all reckon, perhaps, as many more; 350,000 men, however, is a very limited number when spread out from Britain to the confines of Arabia and the Nile cataracts, although only along the Rhine, the Danube, and the Euphrates are there now enemies creating serious military problems.