sink."

"I don't quite see that," replied Jack, with the air of a candid
inquirer. "I have been over the ground, and it didn't strike _me_ so."

"It certainly looks to be several feet lower," said Betterson; and the
boys agreed with him.

"We generally speak of going _down_ to the spring," said Rufe. "We go
down the road, then down the bank of the ravine, and then a little way
up the other bank. I don't know how we can tell just how much lower it
is. We can't see the spring from the house."

"If I had my instruments here, I could tell you which is lower, and how
much lower, pretty soon. While I am waiting for Snowfoot, (I can't go
home, you know, without Snowfoot!) I may, perhaps, do a bit of
engineering, as it is."

CHAPTER XXIII.

JACK'S "BIT OF ENGINEERING."

The boys got around Jack after dinner, and asked him about that bit of
engineering.

"In the first place," said Jack, standing outside the door, and looking
over toward the spring, hidden by intervening bushes on a ridge, "we
must have a water-level, and I think I can make one. Get me a piece of
shingle, or any thin strip of wood. And I shall want a pail of water."

A shingle brought, Jack cut it so that it would float freely in the
pail; and, having taken two thin strips of equal length from the sides,
he set them up near each end, like the masts of a boy's boat.

"Now, this is our level," he said; "and these masts are the sights. To
see that they are exact, we will look across them at some object, then
turn the level end for end, and look across them again; if the range is
the same both ways, then our sights are right, are they not? But I see
we must lay a couple of sticks across the pail, to hold our level still
while we are using it."

The boys were much interested; and Link said he didn't see what anybody
wanted of a better level than that.

[Illustration: TESTING THE LEVEL.]

"It will do for the use we are going to make of it," said Jack; "but it
might not be quite convenient for field service; you couldn't carry a
pail of water, and a floating shingle with two masts, in your
overcoat-pocket, you know. We'll aim at a leg of that grindstone. Go and
stick your knife where I tell you, Link."

Jack soon got his level so that it would stand the test, and called the
boys to look.

"Here! you stand back, Chokie!" cried Link; while Rufe and Wad, one
after the other, got down on the ground and sighted across the level at
the knife-blade