5 22 EXPLORERS' JOURNEYS IN HIGH TIBET.
is only possible on the presupposition that the lake is very shallow and small, so that its entire volume can be renewed every year during the melting of the snows.
On Rawling's map we find a pair of lakes: across the northern lake, the Memar Chaka, is printed the word »salt», across the southern, the Aru-tso, the word »fresh». These are evidently connected by a small watercourse cut through the narrow isthmus that separates them. It is therefore clear that the fresh water in the last-named lake flows northwards into the salt lake, although the Aru-tso is shown on Rawling's map to have an altitude of 16,000 feet and the Memar Chaka an altitude of 16,070 feet. On Deasy's map the relations are however different, and manifestly more in agreement with the actual facts. For the northern, the salt lake, he gives the same altitude as Rawling, but for the Aru-tso he gives 16,210 feet, thus indicating, as might be expected, that the freshwater lake lies considerably the higher of the two. Bower probably exaggerates the altitude of the Aru-tso when he puts it at 17,176 feet. He gives no absolute altitude for the northern lake, nor does he appear to have come directly into contact with it. Whatever the real fact may be with regard to estimates of absolute altitude given by different travellers, one thing is pretty certainly correct, that Deasy's statement with regard to the relative altitudes of the two lakes is most likely to be the true one, and not least because he visited them both at one and the same time ; whereas in Rawling's case a long interval elapsed between his visit to the northern lake and his visit to the southern lake.
With regard to the melting of the snow in this region Deasy gives the following information: »At one camp at the west end of the long valley leading into the lake north of and close to Aru Cho, the small stream by which we camped flowed only for a few hours daily, showing that the hot sun of a summer's day has only a very temporary effect on the glaciers at the western end of the snow range on the south side of the valley.» In any case these lakes owe their existence solely to the precipitation, and the precipitation is more abundant in the summer, especially in its latter half, and the snow too melts only in the summer. During the second half of the 'summer the inflow into the Aru-tso is certainly more copious than at any other season of the year. In case the lake really is, as I have supposed, particularly shallow, the succession of occurrences may be assumed to be as follows. In the spring and early summer the inflow is so insignificant that the evaporation is in excess; the lake is therefore entirely cut off and consequently salt. In the height of the summer, especially in the end of July, the inflow is so copious that the salinity decreases every day. The inflow, finally, reaches its maximum in August, and the surplus water then flows through the connecting river-arm to the Memar Chaka, and in consequence of this the Aru-tso gradually becomes fresh. In proportion as the inflow decreases in amount and the outflow into the northern lake ceases, the lake, being then disconnected, grows increasingly salter. If this order of procedure is correct, it is no longer difficult to understand, that on loth July Bower found the lake salt, that Deasy on the 29th July pronounced it »drinkable», and Rawling on the 29th August found it perfectly fresh. Similar relations, depending upon configuration and situation, are easily conceivable in the case of several other lakes in Tibet.
With this I will conclude my general survey of exploring journeys in Tibet, the object of which was partly to do full justice to the explorations and observa-
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