"Don't worry, Mr. Lang. If our friend the mystery man keeps at us long enough we shall catch him. I wish we knew why he is bothering us so," said Grace. "I should prefer to stay here until we solve the mystery, but we must be on our way, and perhaps he may follow us."
"That sounds interesting," observed Miss Briggs.
Ping and his lazy burros started about an hour before the rest of the party got under way, and when they did get under way they jogged along slowly through the foothills of the range, where the going was fairly easy. The guide said they should come up with Ping before dark, and that they would, after having mess, then continue on at a slower pace until they reached a suitable camping place for the night.
Dusk was upon them when they finally overtook the Chinaman, who was sitting on the rump of a burro chattering to his mount to get him to go faster, but without much success. The ponies of the party then took the lead, which, Hi Lang said, would induce the burros to move faster in an effort to keep up, but it was a much slower pace than the Overland Riders were in the habit of traveling, that they now dropped into.
Night enveloped the outfit suddenly, it seemed to them, and with the cool of the evening their spirits rose. Even Ping's spirits rose, until he forgot his aching thumb and broke into song.
The ground began to slope away under the hoofs of the horses, for they were now moving down a sharp descent, and the air seemed to take on a strange new quality, a new odor. No longer could the girls hear the rustling of foliage. A great and impressive silence settled over them, in which even the footfalls of the ponies were soft and subdued. Glancing up, they saw the stars shining with a brilliancy that none of the party had ever observed before.
The chatter of the Overland Riders died away, and Ping Wing's song died away, also, in a throaty gurgle.
"What is it?" cried Emma Dean. "I feel queer, and my pony is trembling. Oh, Grace, I'm afraid of something."
Grace knew what it was that was disturbing Emma, for she felt something of the same sensation that Emma was experiencing, but she made no reply.
"It is the desert!" answered the guide solemnly. "It is the mystery of the desert, a mystery that no man can solve. Perhaps it is the mystery of centuries; perhaps it is the spirits of the thousands who have perished here on this sweet, cruel sea of burning sand, that have come back to warn us living ones of the fate that may be in store for us who dare."