the meridional routes do lie relatively close to one another, we are not warranted without further criticism in comparing together the absolute altitudes of the various travellers, because some of them determined their altitudes by means of good and reliable instruments, while others have been content to rely upon one or perhaps two aneroids, and in calculating the results obtained have not given themselves the trouble to eliminate as far as possible all the ordinary sources of error. Whereas one traveller has neglected the highly important opportunity of determining the heights of every pass he crossed over, the altitudes obtained by another are in many cases extremely improbable. This is in fact the case with Bonvalot's altitudes, which are palpably too high all through.
It is however hardly probable that each and every • one of the ranges in the interior of Tibet does as a rule decrease in altitude from west to east. It is not conceivable that the passes in the imposing range, for example, which I crossed over at the height of 5462 m. north of Camp XLI 0900, ), increase in altitude towards the west. If that range really does continue the whole way towards the west and connects with the Kara-korum range, it is indeed true that a rise may be observed when we compare the pass in question with the Kara-korum pass, but then the rise is only slight and the fact may be due solely to accidental local circumstances.
To attempt, on the basis of the data that we possess, to estimate the mean altitudes of the different ranges is also a difficult task. If we consider the Astintagh proper, that is to say the section between the bend of the Tschertschen-darja and the western flank of the Anambaruin-ula, we obtain a man pass altitude for the Lower Astin-tagh of 3028 m. and for the Upper Astin-tagh of 3435 m.
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