The great mass of Lannemezan, lying across the threshold of the Val d’Aure, diverts the Neste from flowing north, and turns it to the east, just as the heap of the _lande_ of Pontacq acts at the mouth of the Lavedan, but there deflects the Gave to the west. It falls into the Garonne at the confines of the department, which also for the same reason takes an easterly course for some way, then struggles to the north-east, and only after passing Toulouse turns to take its direction so as to empty itself into the Atlantic. The Neste is a river of very great importance. It rises in two main branches under crests clothed in eternal snows, discharging glaciers into a series of upland lakes. These natural reservoirs have been artificially raised, and their waters conducted into a canal that is carried high above the bed of the river, so as to convey its fertilizing streams over the plateau of Lannemezan. The lake of Caillaouas, under the Pic de Batchinale, and the glacier cirque of the Gours Blancs has been captured at the head of Neste de Luron, and the lakes of Aumar, Aubert, Caplong, and Orredon, that feed the other Neste of Aure have also been utilized for the same purpose, at an enormous expense and by remarkably daring works of engineering. This has had a subsidiary advantage, that the superb scenery at the sources of these streams is now accessible by good roads, whereas formerly it could be reached only by difficult and dangerous mule-paths. At La Barthe the Neste debouches from the mountains through a deep valley, the canal passing above it on the left bank; and although the river has been thus tapped, it still continues to bring down a considerable amount of water, the overflow from its reservoirs far away in the laps of the high mountain ridge. The Val d’Aure constituted a viscounty, and of the viscounts there were several branches: one that of the Viscounts of Larbouste, another that of the viscounts of Asté, one of whom, as already mentioned, married the heiress of Grammont. The whole of Aure was under the overlordship of the counts of Armagnac.