It grew visibly darker, although many of the stars remained twinkling in their places, but they were much encouraged, nevertheless, and trusting in the aid of the night, still saved the strength of their horses. "It will make it a little harder for the Sioux to trail us," said Boyd, "and if, by any chance they should get near enough for a shot, the odds are about twenty to one they can't hit us. Suppose we stop here, give the horses another short rest, and you search the blackness back there with your glasses again." Will was able to discern nothing but the sombre crests of the swells, and Boyd, dismounting, put his ear to the ground. "I hear something moving," he said at last, and then, after a short pause, "it's the beat of hoofs." "Can they be so near as that?" asked Will in alarm. "At first I thought it was the Sioux, but now I'm sure it's running buffalo. I wonder why they're stampeding at this time of the night. Maybe a hunting party of Northern Cheyennes has wandered in here and knows nothing about the presence of the Sioux." "That won't help us, since the Sioux and Northern Cheyennes are allies." "No, it won't. If the Cheyennes meet the Sioux they'll join 'em in the pursuit of us. It's a new danger and I don't like it." Boyd remounted and they rode on slowly. Presently he stopped, and Will, of course, stopped too. "Listen, boy," he said, "and you'll hear the thunder of the buffalo. It's a big herd and they're running our way. I'm as sure as I sit here in this saddle that they're being driven by hunters." Will heard a low, rolling sound like that of distant thunder. It was approaching rapidly, too, and it seemed to his heightened imagination that it was bearing straight down upon them. "If they are Cheyennes we may be in the middle of 'em soon," he said. "If we sit still here," said Boyd, "but that's just what we won't do. We'll gallop ahead until we come to a deep dip between the swells." "And then?" "Dismount, keep low, and let the storm drive by." They did not have much time to spare, as the rumbling sound was growing fast beneath the tread of the flying herd, and they urged their horses into a gallop until they came to a dip, which they thought was deep enough to hide them. Here they dismounted and holding the lariats, watched as the thunder of the running herd increased, until they saw its van of lowered heads, short, curved horns and great, shaggy manes, and then the dark mass stretching back out of sight. "There are tens of thousands of 'em," said the hunter. "They'll be some time in going by, and then, I think, we'll see the Indians hanging on the rear." The multitude drove on for a period somewhat longer than Boyd had predicted, and then Will saw naked horsemen crouched low on ponies, some firing with rifles and others with bows and arrows.