“It is long before the eye can learn to grasp his[72] full dimensions; all around him is rock and mountain, there is nothing to give scale. We are warned of this ... by the camera, for the lens will not look at so wide an angle.... Presently, as we peer over the parapet into the depths of the gulf below us, we realise that there is a man down there walking by the waterside, with a dog that seems to bark though we cannot hear the sound. Slowly our eyes measure the voussoir above which we are standing; it is a twelve-ton block of granite; and the huge vault with its eighty such voussoirs seems to widen and deepen beneath us as we gaze; for the brook that it spans is the river Tagus, whose waters have their source three hundred miles away. “Thus hint by hint we have pieced together the astonishing conclusion that the span of each of the two great central arches is rather wider ... than the interior of the dome of St. Paul’s; and that the height of the railway lines above the Firth of Forth is twenty feet less than that of the road above the Tagus! What must the scene be like in winter, when the waters are foaming against the springer stones one hundred and forty feet above their summer level! How vast the strength of these massive piers which for eighteen hundred years have defied the fury of the floods! “Where now is the great _Via Lata_ that ran from Gades to Rome? Where are the famous cities which it threaded on the way? The vine and olive grow in the forum of Italica, and the Miracles of Mérida are a dwelling for the stork. But here at the wildest point of all its wild journey our eyes may still behold a memorial which nature has assailed in vain: ‘Pontem perpetui mansurum in sæcula mundi’;--the monument of Caius Julius Lacer, more enduring even than Wren’s.”[73]