国立情報学研究所 - ディジタル・シルクロード・プロジェクト
『東洋文庫所蔵』貴重書デジタルアーカイブ

184

THE OSCILLATIONS IN THE WATER-LEVEL OF THE LAKES.

hundred years do we possess a few European observations. The two first years are

unreliable, and the earliest reliable information dates from the Lama surveyors 200 years ago. The years of observation are too irregularly represented to allow us to draw regular curves. In the 18th century there is a gap of nearly 5o years without records, and there may possibly be a depression in the Rakas-tal curve, or in both. In the first half of the 19th century there is a gap of 25 years, and in the latter half there is one of 32 years, of which nothing is known. From the last I o years, on the other hand, there are no fewer than seven observations. This uneven distribution of the years over the period in question shows how difficult it is to extract really reliable laws from the material existing.

One thing seems to be evident : a proceeding desiccation during the last 200 years.

For, disregarding the more numerous observations of later years, the signs become more numerous as we approach our own time. Do these 200 years represent a fraction of the general post-pluvial desiccation, or do they only represent the accelerated change of climate during a depression belonging to a period of lower order? I believe this latter supposition is the right one, and under such conditions we should expect a return to a maximum during the next 200 or 30o years. But the former supposition is not excluded, if we admit that the lakes are such a sensitive instrument, such a delicate self-registering apparatus, that it would permit us to read climatic changes, which are hardly noticeable on ordinary instruments. Such an instrument we find in the movements of the glaciers. But nobody would pretend that the general post-glacial climatic changes could be controlled by readings within 200 years. This would at least not be possible in Asiatic regions. For we know of passes, as the Mus-tagh, which were used in former times, and now are impracticable. The glacial researches in the mountains north of India deliver abundant arguments for general retreat in post-glacial time. But nothing is known about the speed, and the last 200 years are too short a space of time to prove anything. We can only follow the changes in the period of the lowest degree, the advances and retreats during some 5o years.

The oscillations of the Manasarovar and Rakas-tal, on the other hand, should

permit us, if we had the possibility of reading them constantly, to follow extremely closely the curves of at least three different periods. The lowest falls within the course of a year, a rise of the level from the beginning of the rainy season, and a fall from its maximum towards the minimum of the dry season.

The next is the one which may be called the Manasarovar period. It will be well understood if we consider the five years from 1905 to 191 O ; 1905 was regarded as extremely dry; 1907 was very dry with only very occasionally short rains; in 1908 there was a good deal of rain, specially abundant in July and August, and making all the water-courses, coming down from the north to the upper Satlej, grow into rivers, difficult to ford. In 1905 the level of the Manasarovar must have been unusually low; in 1907 it stood, even during the rainy season, 2.263m. below the highest point in