from here
to the bottom of the valley on the other side: eighty or ninety feet. I
should say this embankment is over a hundred in perpendicular height."

"Look here," said Uncle Jack suddenly; "if I know anything about
engineering, this great dam is not safe."

"Not safe!" I said nervously. "Let's get off it at once."

"I daresay it will hold to-day," said Uncle Dick dryly, "but you can run
off if you like, Cob."

"Are you coming?"

"Not just at present," he said, smiling grimly.

I put my hands in my pockets and stood looking at the great embankment,
which formed a level road or path of about twelve feet wide where we
stood, and then sloped down, as I have said, like a railway embankment
far down into the valley on our left, and to the water on our right.

"I don't care," said Uncle Jack, knitting his brows as he scanned the
place well, "I say it is not safe. Here is about a quarter of a mile of
earthen wall that has no natural strength for holding together like a
wall of bonded stone or brick."

"But look at its weight," said Uncle Bob.

"Yes, that is its only strength--its weight; but look at the weight of
the water, about a mile of water seventy or eighty feet deep just here.
Perhaps only sixty. The pressure of this water against it must be
tremendous."

"Of course," said Uncle Dick thoughtfully; "but you forget the shape of
the wall, Jack. It is like an elongated pyramid: broad at the base and
coming up nearly to a point."

"No," said Uncle Jack, "I've not forgotten all that. Of course it is
all the stronger for it, the wider the base is made. But I'm not
satisfied, and if I had made this dam I should have made this wall twice
as thick or three times as thick; and I don't know that I should have
felt satisfied with its stability then."

"Well done, old conscientious!" cried Uncle Bob, laughing. "Let's get
on."

"Stop a moment," I cried. "Uncle Dick said he would show us our
engine."

"Well, there it is," said Uncle Dick, pointing to the dammed-up lake.
"Isn't it powerful enough for you. This reservoir was made by a water
company to supply all our little dams, and keep all our mills going. It
gathers the water off the moorlands, saves it up, and lets us have it in
a regular supply. What would be the consequences of a burst, Jack?" he
said, turning to his brother.

"Don't talk about it man," said Uncle Jack frowning. "Why, this body of
water broken loose would sweep down that valley