At that moment a peculiar cry rang out from a couple of hundred yards or so away--a weird, strange whoop that might have come from some night bird sweeping through the darkness overhead. But it was human, and answered directly by the Baggara train close at hand, and directly after there was a loud shout, and a crowd of horsemen galloped up out of the mysterious-looking gloom, to mingle with the party about to start on their desert ride. CHAPTER TWENTY TWO. NEARING THE GOAL. It was more from hearing than seeing that Frank Frere gathered the fact that the Baggara chief had returned, for after a short pause the camel train was once more in motion, and they were ordered to keep steadily in line in the advance to the desert in the opposite direction to that by which the newcomers had arrived. At first the two parties formed the train alone, for the fresh arrivals had halted to water their horses and camels, quite an hour passing before the sound of approaching horsemen announced that the whole force was in motion, overtaking them at a sharp canter, but only to subside directly into the regular, slow camel pace, which was kept on hour after hour till the dawn, when, looking back, Frank made out that the train extended for nearly half a mile to the rear, being made up of a long line of camels, followed by a troop of many horsemen. It was nearly all surmise, but judging from the number of camels, which were certainly double those that the Baggara had before during their stay by the fountains, they had been engaged in some successful foray, for as the light grew stronger the baggage animals seemed to be very heavily laden. This idea naturally suggested that the wild horsemen had been engaged in some desperate encounter, and half laughingly the professor bantered his friends about their prospects. "It means a revival of professional practice for you," he said, "and that looks prosperous. You only lost your last patient a few hours ago--that is, if you have lost him--and now a score or two will come tumbling in." "Very well," said the doctor coolly; "it shows that they approve of my treatment. I suppose we shall know at the first halt." This was many hours in coming, for a long, monotonous march was made right away to the south-west, with the pile of rocks they had left gradually sinking till quite out of sight, and then, with the sun growing hotter and hotter, there was nothing visible on any side but the long, level stretch of sand. The halt was not made till near midday, when the heat had become unbearable, and horses and camels were growing sluggish, and showed plenty of indications of the need of whip and spur.