"What would happen if they ran dry?" David interrupted. "Could they get their water anywhere else?"

"They never have run dry, lad, and I don't reckon they ever will. But if they all did, 'twould be very serious indeed, for the cities can't do without us. But they never would run dry all at the same time because we never use 'em all at once. That's what a reservoir is for - to keep the water till it's needed."

"What would happen if you overflowed?"

"'Tisn't likely we would. Water couldn't come in faster than I can let it out through these pipes. Each reservoir lets out as much as is needed at the water works. Hatchholt was an experiment though, because the flow is controlled at this end. Not all the other reservoirs have pipes and are as high or as isolated as this one... Come in here and I'll show you the sluices..." and he led the way to the little brick house and unlocked the door. There was not much to see. The floor was a metal plate obviously covering some sort of well, and through it projected four iron rods each with a small wheel and a dial.

"Each of these wheels is like the handle of a tap," Mr. Sterling explained. "Turn them to the right and the sluice about twenty feet down opens and the dial shows how much water is running through. Turn 'em to the left and they close just like putting the plug in the bath. I don't rightly remember that we've ever had all four open at the same time."

"Suppose," said David. "Just suppose that the pipes got blocked somewhere or the sluices wouldn't work and the water kept coming in and you couldn't get it out. What would happen? Would the dam burst?"

"Lord bless you, no! That wouldn't break! Water would pour over the top and run away down into the valley making a stream like that in your valley, or Dark Hollow."

Then he allowed David to open and close the sluices and watch the dials and continued to chat amiably while Peter prowled about furiously outside. When at last he had had enough and Mr. Sterling had ambled away, David remembered Peter and turned to find her. She had disappeared, so he went back to the house. She was not there either, so he collected his bathing shorts and towel and went up to the dam again and whistled the peewit's plaintive call.