railway station. If stacks of what after all is mere straw could without any special protection withstand the destructive effect of two thousand years, the climate and conditions of the Tun-huang desert may well be credited with preservative qualities of an exceptional order. The use made successively of the stored materials would account for the greatly varying height of the stacks from one to seven feet still extant at the same watch-station.
But it still remained to explain why some of the stacks at different posts were found reduced by fire to calcined fragments. It was easy, of course, to think of wilful damage done by raiders and the like. But the most plausible explanation did not suggest itself until M. Chavannes' translations showed me how frequent are the references to fire signals in the records from the watch-stations. No doubt such signals would ordinarily be lit on the top of the tower. But when time was pressing, or perhaps a particularly big fire was needed to penetrate a murky night, it would be simpler to set a whole stack on fire. The fact that the remains of burnt stacks were always found at points of the quincunx where the risk of igniting others was less, supports this interpretation.
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