One above another these vineyards extend along the lake to the height of two thousand feet. They are formed with persevering industry upon these precipitous slopes by means of parallel walls, whose narrow intervals are filled with earth that has been carried up by the peasants in baskets upon their backs from below; and in the same way they must every season be abundantly covered with manure. These successive terraces are reached by steps frequently cut with infinite labor in the hard rock, and with every economy of the land. Every inch of the ground is valuable, because only on the side of certain hills will these vines come to perfection. These lands, after being purchased at so high a rate, need constant attention; for the soil is washed away from these steeps beneath the stone walls, and must be replaced every spring; every clod of earth is a great treasure, and they carefully collect the earth which has been thrown out of a ditch to fill up their vineyard patches. A square foot of land is reckoned to produce two bottles of wine annually. Every portion of a vine is used; the stems and leaves serving as food for the cattle; the husks, after being pressed and wedged into round moulds, then dried, are used for fuel, burning something as peat does. In many houses of this section the cellars are enormously large, with a capacity as high as a million bottles each; and they are often used as the common sitting- and reception-room.[80] Vineyards also flourish on the slopes surrounding the lakes of Neuchâtel, Biel, and Zurich; in the valleys of the larger rivers and certain plains of northern Switzerland they are found to a small extent. Still, the wine produced is not sufficient for the demand, and over 15,000,000 gallons are annually imported; in this consumption the great number of tourists who come every season must be taken into account.