Just here the Wall crosses the Knag Burn, and it is interesting to note how it crosses, because it was no doubt the method employed by the Romans to carry the Wall over every narrow stream. First, the bed of the stream has been paved with stone; then low walls, four or five courses high, have been built along the edges of the stream for a distance of about 10 feet. Lastly, large slabs of stone have been thrown across from wall to wall, to bridge the stream to a width of 10 feet, and on this foundation the Wall has been built. It leaves an opening for the stream about 34 inches high, 30 inches wide, and 10 feet long, not a very pleasant passage for one man to squeeze through, even when the stream was dry, and hopeless for a raiding band! At this point a notice is posted: "Admission 6d.; parties over 20, 3d. each." It reminded me of the old lady who claimed to be so much over twenty that she ought to be admitted for a penny! The Wall joins the north wall of the fort at its rounded north-east angle. {119} CHAPTER XII HOUSESTEADS TO PEEL CRAG BORCOVICIUM The fort of Borcovicium was constructed on the same general plan as that of Cilurnum, but it presents also a very great contrast in situation and in its special features. Cilurnum lies in the fertile valley of the North Tyne; Borcovicium clings to the bleak heights; but they are alike in the massive nature of their gateways, in which pivot-holes and wheel-ruts can still be seen, and alike in the general arrangement of the Principia and other buildings. At Borcovicium the Principia faces east, instead of north. A wide arch covered the main entrance, and there were two similar arches inside. There was the usual outer court, surrounded by a colonnade which supported a pent-house roof; the inner court, with a portico; and the series of five small rooms at the back. In the northernmost room were found over eight hundred iron arrow-heads and some scrap iron, as if {120} some one had been making arrow-heads here before the fort was finally deserted. The columns here are round, as contrasted with the square piers of Cilurnum. One very beautiful base is left; it was probably turned on a lathe, as a pattern, and then the building appears to have been interrupted, and the other bases were copied from the first, not very successfully. The storehouses (_horrea_), for supplies to last all the winter, have had as usual very thick walls and buttresses, in order to support the heavy stone roofs, tiled with stone slabs, to prevent their being set on fire by red-hot sling-bullets. The floors were raised above the ground on squared blocks of stone, to keep the buildings dry, and so preserve the grain. In the Middle Ages a circular kiln was made in the southern granary.