"That was caused by the feathers being placed in a sort of winding way, so as to make the arrow whirl as it flew," Dick explained. "It made me think of the duck we call a whistler, whose wings make a sound as it flies such as you can make by blowing in a hollow reed. But we seem to have outdistanced the enemy now, and perhaps it would be safe to breathe the horses a little." They pushed on during the whole of that day without once meeting any difficulty; although there were times when they found themselves compelled to make wide detours in order to avoid bad stretches of ground, or sloughs, where the footing was treacherous for the horses. (Note 3.) On this account they did not make the progress impetuous Roger would have wished for, although his companion declared himself satisfied. "Better luck, perhaps, to-morrow," he would say, whenever Roger complained; and thus the latter was shamed out of his mood. That night they found a place to camp that was totally unlike the fortress amidst the rocks, but offered them just as secure a refuge. And again they saw daylight come without any alarm. So three days passed away, and it was now to be hoped that they would see nothing more of the half-breed. Dick remained on his guard, all the same, for he did not mean to be caught napping. The nature of the country had changed again, and, instead of the woods or rocky bluffs which they had left farther down the river, they now found themselves looking out on vast stretches of level prairie, where the tall grass waved in the breeze until it resembled the waters of a wide sea; and in places innumerable wild flowers dotted it like splashes of paint, making a picture that even boys could admire. Here they would be apt to come upon many novel things, of which they may have heard wandering trappers speak, but which up to recently they had hardly expected to see for themselves. At the same time there would be ever-increasing danger of their being discovered by some hand of red hunters, stalking antelope or bison, and ready to leave their hunting for a more convenient season if they saw a chance to capture palefaces, with their wonderful "shooting-sticks," which in those days were a source of great bewilderment to the Indians west of the Mississippi. But nevertheless, in spite of the constant presence of peril, the two lads enjoyed the experience, and had no regrets about having started on the adventurous mission, since they were at the same time serving those they loved so well, and satisfying the craving for excitement that seems to be a part of almost every boy's nature. CHAPTER XIII ON THE BILLOWY PRAIRIE