“No,” said Uncle John, “but it helped to make it cool. If you want water to form out of vapour, you must give it something cold to form on. Breathe on a cold window-pane, and see how the tiny drops of water settle on the cool pane from the water-vapour in your breath. If you were to take a cold basin and set it out of doors at night when the dew is falling, you would soon find the drops of water trickling down its sides. “This is just what those people did, only theirs was a larger plan. “They made their pond, as we read, and finished it one day before evening. Then what happened? All day long the heat of the sun had been warming the ground round about, but it could not warm the thick moist clay so much as it warmed the turf of the hill-side. “Then, after the sun went down, everything became cooler. But the clay pond was still the coolest thing there; and the packing of reeds and brushwood kept the heat of the earth from passing into it from below. “So the dew began to settle in drops upon the cold clay and upon the smooth stones, and it trickled down the sides. As we said, there is always plenty of moisture in the cool night air; and all you need to do is to provide the proper place for it to collect in. So it was with the dew-pond in our book. “One by one the drops formed and ran together, as soon as they had found something to run into; and millions and millions more joined in, coming as vapour and settling down as water until the pond was full. “And so long as the clay bottom of the pond kept whole and sound, the dew-pond would hold water, making up at night what it lost by day. But if once the clay were broken or worn through, the water would run away into the ground, and the pond would never fill up again; partly because it would not hold water, and also because once the brushwood became soaked, it would fail to act in keeping the clay cool.” “Was that why the first pond failed?” asked Dick. “I expect it was. I expect the clay bed had in some way become worn through, so that the pond would not hold water. And the people guessed quite rightly that the only way to mend matters was to make a new pond, though, as Joe says, they gave an odd sort of reason for it.” “Do people make ponds in that way now?” Joe asked. “Yes, I believe so; though not many people know much about it, since there are so many other ways of getting water possible nowadays. But I have heard that in some parts of the country there are old men who know how to make dew-ponds.” “Do any of the dew-ponds that those people made exist now?” Dick asked.