National Geographic : 1946 Nov
The Roman Army Crosses Alcintara in Spain ROME held her empire together by magnificent roads over which she moved her administrative officials and her garrisons into the remotest of her provinces. To shift her armies with maximum speed, she relied on the broad, straight, .all-weather highways, traces of which still exist along a 4,000-mile track from Scotland to the Persian Gulf and on the even longer stretch from Morocco through Egypt to Turkistan. In constructing these highways, the engineers often se lected a conspicuous landmark on the horizon and ran the road toward it with surprising disregard for topography. Cuts and fills, diked embankments, viaducts and bridges, and tunnels all contributed to preserving a straight direction and an even grade. The roadbed was always excavated to hardpan or solid rock. Layers of rubble and coarse concrete packing carried a surface of closely fitted paving blocks. Curbs and shoul ders and drainage ditches were added; and the result long outlasted the empire which created it. The great bridges over the larger river valleys were the most spectacular feature of the military network of com munication. Initial responsibility for their construction must have lain with the military commanders and provincial governors acting in the emperor's name; but local com munities were not slow to copy these higher authorities. In A. D. 105 the eleven communes inhabiting the hill country where the chief river of Spain, the Tagus, crosses the Portuguese frontier petitioned Trajan to be allowed to defray the cost of a bridge across the river. It was built entirely of granite blocksset without use ofmortar. The six arches are opposed twoand two, with theshortest under the approaches and the broadest pair with 90-foot spans carried on piers set in midstream. From thefooting ofthe piers in the river bed tothe roadway these arches carry, the height is nearly 200 feet. An army could march across with eight men abreast, and 2,000 troops could be crowded onthebridge atonetime. In marching order, however, alegion with itshorsemen, artillery, and baggage trains would spread outformore than a mile, its van disappearing around thespur ofthe opposite hills before the rear guard reached theriver. Three such legions were usuallystationed inSpain. The Roman bridgewaysoften carried arches attheend or, as on Alcantara, at themiddle ofthespan. These could be fitted with tollgates orwith heavy barriers tocheck unwelcome passers. Alcantara was blown upbytheBritish under SirArthur Wellesley (later the DukeofWellington) 'inthePeninsular War, 1809, and by the Carlist insurgents in1836. The arches were repaired, however, and thebridge stands inuse today, one of the grandestofallRoman remains. The ballistae and catapults being hauled across thebridge have been dismantled for transport. Neither, ofcourse, ever carried an explosive charge,butrelied onthesudden release of tightly twisted thongs made ofhair orsinew. Even so, such crude artillery was sufficiently feared and effective in siegework to be worth transporting onoxcarts ormule drawn wagons along withthe marching legions.