"We'll do it whether they let us or not!" exclaimed the engineer. "It's my fault, in a way, that they did this, for I pointed out the advantage it would be to them to have a dam, and I'll do my best to make good the trouble caused. Come on, Dave. Well ride up above and see what we can do. Meanwhile, you boys do your best to keep the cattle from stampeding. They won't let themselves be driven away, that's sure, so we've got to bring water to them." "If we only can," murmured Cowboy Dave. He felt it to be a hopeless task. Now that the cowboys had given over their efforts to drive the cattle away from the water-hole the beasts were quieter. Left to themselves, they scattered somewhat and sought for places where little pools might have formed, and where they could slake their thirst. It was not much water that remained, and the bellowings of the cattle, and their panting appearance as their parched tongues fairly hung from their mouths, filled the hearts of Dave and his friend with pity for the poor beasts. "We haven't any time to lose," said Mr. Bell-more, as he urged his horse, Kurd, toward the bend of the stream. Dave, on Crow, followed, wondering what would be the outcome. Dave glanced back from time to time at the thirst-maddened cattle. Some of them forced their way into the muddy sloughs in spite of the desperate efforts of the cowboys to drive them back. Then it was necessary to try to pull them out by lariats attached to them, and extending to the horns of the saddles. "Poor beasts!" murmured the young cow-puncher. He and Mr. Bellmore had ridden for perhaps a mile when they saw a figure galloping toward them. "Who's that?" asked the engineer, pulling up suddenly. "Dad," answered Dave. "He rode up to investigate. He'll be able to tell us what happened." "It's easy enough to guess," said Mr. Bellmore. "The Molicks have built an intercepting dam, to get even with you and me." And this was exactly what Mr. Carson reported as he rode up. "They've cut off our water supply," he called to Dave and the engineer. "They've made a board and mud dam right across the river, and turned the water onto their own land. They're making irrigating ditches now as fast as they can work." "I suspected as much," Mr. Bellmore said, "Is the dam a very big or strong one?" "Not especially so. But the water is low at this season of the year, and it doesn't take much of a dam to block it off from me. This dam is made of boards, banked up with clay and stones." "Would it be easy to break away?" asked the engineer. "Yes, I suppose so. But Molick will take precious good care that it doesn't break away, They're strengthening it all the while." "Oh, I didn't suppose it would break away of its own accord," the engineer said. "I meant would we have much trouble in making a breach in it?" "We?" cried Dave. "Do you mean--" "I mean that we've got to break that dam to save your cattle!" the engineer said. "It's the only way!"