“And I don’t understand why they put the bundles of fern and brushwood underneath,” said David.
“It isn’t at all hard to understand, really,” said Uncle John. “You see there is always a great deal of moisture in the air. Sometimes it is high up, and then we call it clouds; and sometimes it is low down, and we call it mist or dew. But there is always plenty of it—a never-ending supply. Of course you know how it comes to be there?”
“The sun draws it up.”
“Yes, the sun draws it up. The sun is always sucking up water from the earth, from the sea and from rivers and lakes and ponds, and from puddles in the road and from clothes hung out to dry. And the warmer the sun is during the day, the more water it sucks up into the air. But in the night when the air is cool, some of this water comes back again: it forms into drops and settles on the grass or on cabbage leaves, or on a book which you may have left out all night on the garden seat. You know that if you go out early in the morning and walk in the grass, you get your boots sopping wet. So if you could find a place that was hollow so that water could gather in it; and if you could keep it cool like a cabbage-leaf, so that the water would settle in it; and if you could make it watertight below so that the water wouldn’t leak out, you would have a dew-pond.”
“Why should it be on the top of a hill?” Dick asked.
“I suppose because the higher up you go, the more chance you have of getting into the clouds and the moist air; dew falls more abundantly on the sides of hills.”
“But why did those people put the bundles of fern and stuff under the clay?” asked Joe. “That didn’t help to make it water-tight did it?”
“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.