outlet 62 can be orientated and encouraged by particular surface states in certain zones of the sheet 24 thus in fig5 and only by way of illustration are shown two zones 64 the surfaces of which are perfectly smooth of mirror polished type and two zones 66 the surfaces of which are grained the flow is facilitated in the mirror zones 64 and thus the stagnation of the liquid 20 is prevented in the grained zones 66 the flow is on the contrary slowed which permits the avoidance of large drops 20 arriving too quickly at the ribs 54 58 or the channels 56 and passing over which would cause a cascade effect incompatible with the precise guiding of the flow the sheet of fig4 is rectangular and symmetrically divided into six zones themselves virtually rectangular three upper zones left hg central hc and right hd are situated just above three lower zones left bg central bc and right bd the dividing lines between these zones are formed by compression barriers 52 extending in a straight line in addition vertical barriers 52 extending from side to side of the central zone bc define two outlet corridors 68 the upper zones hg hc hd all have a slight opening in the barrier 52 forming their lower dividing line and at this position each barrier 52 adopts a slight slope p so as.