39 8 THE POPULAR SCIENCE MONTHLY. 



been collected since, and in the hands of Mr. Buchan they have 

 undergone such a careful and able analysis that the Challenger 

 Reports charts may be taken as the best reliable representation of 

 the winds, the temperatures, and the pressure in the lowest strata 

 of the atmosphere, a well as the surest basis for further generali- 

 zations.* The theories which have been mentioned in the preced- 

 ing pages give the grand lines of atmospheric circulation; on 

 Buchan's maps we see how the grand lines are modified in the 

 lowest strata by the distribution of land and sea, and the unequal 

 heating or cooling of continents and oceans. The leading features 

 indicated by theory are still maintained, and they become even 

 still more apparent if we consult isobars traced for a certain 

 height, like those of Teisserenc de Bort ; but the immense plateaus 

 of East Asia and North America act in winter as colossal refriger- 

 ators, where cold and heavy air accumulates, to flow down in all 

 directions toward the lowlands. We see also how in 'July the air 

 is heated in the lower lands of northwest India, in the corner be- 

 tween the Afghanistan and the Thibet plateau, how pressure is 

 lowered there by the ascending current, and how winds blow 

 toward this region of lowered pressure. We see more than that : 

 on looking on the maps it strikes the eye how the moisture or the 

 dryness of the climate is dependent upon the distribution of press- 

 ure, and how the dry anticyclonic winds make barren deserts of 

 parts of North and South America, of Africa, and central Asia, 

 and how they will continue to dry the lakes and the rivers of 

 these regions and occasion total failures of crops so long as that 

 distribution of pressure lasts on the globe, and man has not yet 

 learned to eschew its effects by getting water from the depths of 

 the earth. The life of the globe\ during the present period is 

 written on these splendid charts. Nineteenth Century. 



M. THOEADDSEN, in the narrative of his travels in Iceland, observes a peculiar 

 feature of the oases at the foot of Mount Hecla. These oases are subject to con- 

 stant displacement by the violent sandstorms which are common. On the wind- 

 ward side all vegetation is gradually destroyed, while on the other side grass takes 

 root, and in a wonderfully short time the level and sterile surfaces are converted 

 into good pasture lands. 



made since, Mr. Buchan's isobars having been one of our best arguments to press the neces- 

 sity of the leveling. But Mr. Buchan may not be aware that the leveling beyond the 

 ninetieth degree of longitude is now considered by Russian geodesists as utterly unreliable; 

 it is supposed to contain some substantial error, so that a new leveling between Krasno- 

 yarsk and Lake Baikal is insisted upon. The incertitude in the isobars on an immense space 

 in northeast Asia resulting from this cause may attain as much as one or, perhaps, even 

 three tenths of an inch. 



* An excellent resume of the whole work and its results in a popular form has been 

 published by Buchan himself in the Proceedings of the Geographical Society, March, 1891. 



