Perhaps I can make the thing clear in this way. When Good and I were last in London together we went to Madame Tussaud’s and saw the famous guillotine that was used in the French Revolution. The knife of that guillotine, you will remember, was raised between uprights, and when brought into action, let fall again to the bottom of the apparatus, severing the neck of the victim in its course. Now imagine that those uprights were the rock walls of the pit, and that the knife, instead of being but a narrow thing, were a great sheet of steel, or rather stone. Then, when it was drawn to the top of the uprights from the niche at the bottom, it would entirely fill the space to the head of the grooves, and none of the water that normally passed over it could flow between the uprights, or rather walls, because the sheet of stone barred its passage. Now do you understand? As Good, who was stupid about such matters, looked doubtful, Allan went on: “Perhaps a better illustration would be that of a portcullis; even you, Good, have seen a portcullis, which, by the way, must mean a door in a groove. Imagine a subterranean or rather subaqueous, portcullis that, when it was desired to shut it, rose in its grooves from below instead of falling from above, and you will have an exact idea of the water door of the priests of Heu-Heu. I’d draw it for you if it wasn’t so late.” “I see now,” said Good, “and I suppose they wound the thing up with a windlass.” “Why not say with a donkey engine at once, Good? Windlasses had not occurred to the Walloos. No, they acted on a simpler and more ancient plan. They lifted it with a lever. Near the top of this slab of rock, or water door, was drilled a hole. Through this hole passed a bolt of stone, of which the ends went into the cut-out base of the lever, thus forming a kind of hinge. The lever itself was a bar of stone—evidently they would not trust to wood which rots—massive and about twenty feet long, so as to obtain the best possible purchase. When the door was quite down in its niche at the bottom of the bed of the channel, the end of the lever naturally rose high into the air, almost to the top of the pitched roof of the shed, indeed.” When it was desired to raise the door so as to regulate the amount of water passing into the irrigation channel beyond, or to cut it off altogether in case of flood, the lever was pulled down by ropes that were tied to its end by the strength of a number of men and that end was passed into, or rather under, one or other of half a dozen hooks of stone hollowed in a face of solid rock. Here, of course, it remained immovable until it was released, again by the united strength of a number of men, and flew back to the roof, letting the portcullis slab drop into its bed or groove at the bottom of the channel, thus admitting the lake water.