In the foregoing chapter my sole purpose was to give a general idea of the hypsometrical relations of the Tibetan highlands. For the details along my own routes I refer the reader to the Meteorological Journal in vol. V. Once more however I would emphasise what I have already said, that my attempt is to be regarded merely as tentative, being based on very scanty materials. And the same opinion must be pronounced, I am sorry to say, upon other general accounts of the physical geography of Tibet. As promised at various places in the third and fourth volumes of this work, it was my original intention to put together a comprehensive survey of the physical geography of the country; but I have come to the conclusion that it can only be to the advantage of any such survey to postpone it for a few years longer. At any rate, that is the case as regards myself personally, and for this there exist several reasons. In the first place the assimilation and study of the great mass of cartôgraphical material which I brought home with me from Tibet are not yet finished. The maps constructed on the scale of i : 200,000 are to be used as a basis for the general map of northern, eastern, and central Tibet on the scale of i : i,000,000, of which I have spoken before. This map will not therefore embrace southern Tibet, that is that most interesting and mysterious country around Nain Singh's lakes and the valley of the upper Brahmaputra, as well as the country north of the Himalaya. It is for the purpose of filling up this gap that in November 1905 I purpose starting upon a new journey. I ask myself therefore, what object would be served by attempting a comprehensive account, seeing that if the same good fortune attends my new journey that has smiled upon the old, I shall within a short time be in a position to produce a comprehensive work such as I have described, but in a more complete way than is at the present moment possible. After I return from my contemplated journey I shall not only have at my disposal far more copious materials, but I shall also find my general map, so often alluded to, ready for me; besides which, I shall then be myself in possession of that general view of those parts of Tibet which I visited in 1900 and 1901 which I now lack. Without that map it would, for instance, be labour in vain to attempt to trace out the line of the water-partings throughout the whole of the internal-drainage area of
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