Next day, whilst we were resting at Camp XXIX, the same cycle was repeated, except that the northerly wind began as early as 5 p.m., and there was no antecedent calm. The westerly »trade-wind» veered round gradually and imperceptibly to the north, but did not attain its full force until 7.3o p.m., when it blew with a velocity of 16.9 m. in the second. At the moment it began the air was full of extraordinarily fine dust, so that the nearest mountains loomed through it like dim silhouettes or flat walls. Everything that lay loose on the surface of the earth was carried away, so that on the following morning the locality of our camp looked as
though it had been swept clean with a broom. Thus here also, during the short
intervals in which the ground is quite dry, the wind carries on its excavating work, though, as we have seen, that does not in general amount to much, owing to the fact that in the warm season the ground is generally moist and in the cold season is frozen. Yet however seldom it happens that the ground becomes as dry as it was in the vicinity of Camp XXIX, so dry as to fall a ready prey to the wind, nevertheless this circumstance does, in however slight a degree, serve to retard the filling up of the latitudinal valleys and basins, for the transporting power of the wind is naturally greater in the bottoms of the latitudinal valleys than on the mountain ranges that separate them, consisting as they do of hard material and covered as they indeed often are with snow. Perhaps the configuration of the country, when considered as a whole, plays a part which ought not to be overlooked. Camp XXIX lay in a relatively deep depression (4959 m.), bordered on the north by a range that rises to 5337 m., pass altitude, and on the south by a range that reaches 5210 m., pass altitude. Both these ranges should screen the intermediate latitudinal valley against precipitation, and as a fact the ground in it was considerably drier than it was both north and south of the same locality. Here too we came across small strips of drift-sand, though they were, it is true, exceedingly small, whereas drift-sand is entirely wanting in the moister regions that are more directly exposed to the precipitation. With regard to the grouping of this drift-sand, it did not escape my observation that it was always accumulated on the western slopes of the glens: that is to say in the meridional glens that slope down towards the south it is heaped up on the terraced escarpment on the right, while in those that run down northwards it is found on the left side. This arrangement itself tells us that the prevailing wind must blow from the west, as also that the night wind from the north which I have mentioned above is unable to effect any visible alteration in the arrangement of the sand. This suggests again, that the reason why this northerly wind plays such an unimportant part is that it blows for such a short portion of the year, whereas the westerly wind prevails nearly all the year round. With regard to the period of the rain, this coincides, so far as my experience goes, at all events in middle and southern Tibet, with the late summer; and the same would appear to be the case, though in a less pronounced degree, in the northern and eastern parts of the highlands. On this occasion we had got up on to the high plateaus a good deal earlier than we did the year before, and the precipitation then was certainly more copious than it was on this present occasion. During my journey through northern Tibet in the year 1896, which fell in the latter part of the summer and in the autumn, I observed that there was a plenteous precipitation. And even in the year 190 I, during the
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