A plan and vertical section of the leaches is shown in Fig. 78. Assuming that the leaches have been working for some time and that the liquor in the strongest leach has been run off to the tan-pits, or in the case of manufacturing extracts to the decolorising tanks or evaporator, the last vat in the series is now filled with water or spent liquor, which may be heated by steam if desired, and this water, which completes the exhaustion of the material in this vat, forces the liquor forward in the whole series, so that it gets stronger and stronger as it passes from vat to vat. The very weak liquor remaining in the last vat is now pumped into a spare pit, or on to the next stronger vat, pressing the liquor forward as before; the vat is emptied of the spent material and refilled with new, and now becomes the head leach; and the strongest liquor is pressed on to it by running water or weak liquor on the weakest vat. As regards the construction of such a “battery” of leaches, details will differ according to whether the usual English square sunk pits, or the American form of circular tub-leaches is employed. In the former case the vertical spouts connected with the space under the false bottoms are usually made of wood, like the old fashioned “eye,” and placed at one side or corner of each pit, and connected with the top of the next pit by a short trough which may be open above or covered as preferred. Both eyes and cross troughs must be of ample size, so as not to check the running of the liquor, and for a set of six or eight leaches, the bottom of the cross trough should be at least 10 or 12 inches below the actual top of the leach, which should not be filled with material above that level. The object of this is to allow of a sufficient fall from the first to the last leach. Means must be provided for the temporary closing of the cross-trough between the vats which form the first and last leach. On a very small scale, this may be done with a plug; sliding wooden doors are convenient, but difficult to keep tight. A hinged or sliding door held against an indiarubber facing by a wedge or toggle-joint would seem a practicable device. If round tub-leaches are employed, the vertical connection may be similarly made with a wooden trough, but copper tubes are almost essential for the cross connections. If a vertical copper eye in the centre of the leach be provided for boiling, or for emptying the leach (p. 334), it may be utilised for the upflow by connecting it with the cross pipe with a thin copper pipe of large diameter, which must be movable for the purpose of casting the leach. A joint like that of a stove-pipe will probably prove sufficiently tight, but if necessary may be made tighter by rolling an indiarubber ring over it.