That afternoon, dressed in new overalls and blouses, with a big, good-natured colored man to help with the laboring work, the boys were early on the job, at first making a cement mixing box; then Bill drove the center stake thirty feet below where the dam was to be placed and from which, using a long cord, the curve of the structure twenty-nine feet wide, was laid out upstream.

At the spot chosen the rock-bound hillsides rose almost perpendicularly from the narrow level ground that was little above the bed of the stream; it was the narrowest spot between the banks.  George, the colored fellow, was set to work digging into one bank for an end foundation; the other bank held a giant boulder.

The boys were giving such close attention to their labors that they did not see observers on the hilltop.  Presently the gruff voice that they had heard before hailed them from close by and they looked up to see Mr. Hooper and the slim youth approaching.  The boys had heard that this Thaddeus was the old man’s nephew and that he called the Hooper mansion his home.

“What you drivin’ that there stake down there for?  Up here’s where the Perfesser said the dam was to set,” Mr. Hooper demanded.

“Yes, right here,” Bill replied.  “But it is to be curved upstream and that stake is our center.”

“What’s the idea of curvin’ it?”

“So that it will be stronger and withstand the pressure.  You can’t break an arch, you know, and to push this out the hills would have to spread apart.”

“I kind o’ see.”  The old man was thoughtful and looked on silently while the dam breast stakes were being driven every three feet at the end of a stretched cord, the other end pivoting on the center stake below, this giving the required curve.

“How deep you goin’ into that hill?  Seems like the water can’t git round it now.”  Mr. Hooper, at a word from Thad, seemed inclined to criticize.

“We must get a firm end, preferably against rock,” Bill explained.

“Shucks!  Reckon the clay ain’t goin’ to give none.  How much fall you goin’ to git on that Pullet wheel?”

“Pelton wheel.  About eighty feet, Professor Gray figured it roughly.  We’ll take it later exactly.”

“Kin you improve on the Perfesser?”

“No, but he made only a rough calculation.  We’ll take it both by levels and by triangulation, using an old sextant of the Professor’s.  It isn’t a diff——­”

“What’s try-angleation?” Mr. Hooper was becoming interested.

“The method of reading angles of different degrees and in that way getting heights and distances.  That’s the way they measure mountains that can’t be climbed and tell the distance of stars.”

“Shucks, young feller!  I don’t reckon anybody kin tell the distance o’ the stars; they only put up a bluff on that.  They ain’t no ackshall way o’ gittin’ distance onless you lay a tape measure, er somethin’ like it on the ground.  These here surveyors all does it; I had ’em go round my place.”