"But, Mr. Field," said Roger to his chief, when this was explained to him, "if a drainage ditch were cut direct from the highest point of the marsh to the Red Lake River, would not all the water naturally flow into it, and so drain the swamp without all this elaborate surveying?" "And how would you find the highest point or points of the marsh," said the other, smiling, "without a survey? You see, son, this swamp is like a continent on a small scale. It has its mountains and its valleys, its plateaus and its ravines, though these be measured in inches instead of hundreds of feet. Now, if this ground were rocky, all this drainage would make for itself a network of small streams and flow down to the river, but as the ground is naturally spongy the water has lain instead of running, and therefore has not cut any channels. Add to this the hundreds of thousands of years' deposit of rotting vegetation, and you see how impossible it is for the water to do what would naturally be expected, that is, find its own level." "But it must flow down some time, surely," said the boy. "The overplus does. In spring, that is to say in early spring, right after the snow melts, this whole swamp is a sheet of water, even worse than it is now, and the houses on the higher grounds are on islands, the farmers going to and from them with boats, but that soon runs off until it reaches the level of complete saturation, in other words, a bog as wet as it can hold. Now, what we have to do, is to trace this highest point or points, such as you spoke of, or, to speak more correctly, the succession of the lines of highest points, a very crooked series of lines, and find out their relation each to the other. This you see, will divide up the swamp into several drainage areas. Then each of these areas is to be surveyed to determine the line of drainage, the whole to be conformed to the main ditches that will flow to the river, and this intricate network of ditches must be kept at just the exact level of fall, so that it will flow unencumbered to the streams on either side of the swamp." Roger whistled softly. "That's why you've got to go over every foot of the country so carefully," he said. "Of course. If it wasn't for the trees and brush, which prevent us seeing just where every little rise is, it would be comparatively easy, but unless we know the lie of the ground, we might plan a ditch just on the wrong side of a ridge of comparatively solid earth, which would divert the entire stream. Of course, there's a pretty good fall to the river, both the Mud River and the Red Lake River, but even so, an unobserved ridge of earth a few feet high, running along for a couple of miles would throw out the value of that particular ditch and create the cause for a new drainage area." "I see," said the boy, "and I'm very glad you told me, Mr. Field, because it did look to me as though a lot of this exactness was unnecessary."