be lost. But when a thorough search of all our kit at the lake encampment failed to find the precious book of figures, I knew that latitudes also must be rare.
Even a very exact determination of position would not have given us a trail, but could have determined general directions toward an objective and the distance to be traversed. As things were, we had nothing save compass readings for guidance. My instrument was small, not well made, and I did not know the magnetic variation on the Tibetan plateau. Experience had taught me in other journeys that results, sometimes remarkably accurate, may be had by compass work, assuming an average rate for caravan speed. This must vary with the animals used. Thus, Somali camels go steadily at about two and a quarter miles per hour; Abyssinian mules may be counted to do three miles per hour over anything but very rough country. Our Turkestan ponies, as we had determined on the lower desert, were good also for three miles. And this figure was, for a time, assumed on the plateau, making specific allowance for all stops over one
minute. It proved to be too high, the animals
being slowed down by the rarified air and equally rarified food.
During the first five days beyond the pass the error of magnetic variation was of small account, as our course had been generally southward with approximately equal east and west diversions. It became serious on the long westerly course soon to be pursued. The compass course pointed a wavering and inaccurate path across the untracked wastes.
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