1865.] Nvtes on Central Asia. 127 



sheltered ravines or defiles which are hardly reached by the rays of 

 the sun. Had I fallen into these errors in my determination the 

 results-would have been to lower instead of to raise the height of the 

 snow-line, as compared to its true limits. But these sources of error 

 were fully anticipated and averted ; my observations were made at 

 points where regular layers of eternal snow occurred, and moreover on 

 mountain-ridges and not in hollow depressions, in some of which 

 I really did find eternal snows in some cases several hundred feet 

 below the limit of 11,000. 



"With regard to the other point, I must observe that the method of 

 determining heights by the temperature of boiling water, is certainly 

 one which is far from being perfect ; and leads only to approximate 

 results ; but the inaccuracy of these results becomes more inappreci- 

 able, the greater the height which is being measured. For inconsi- 

 derable elevations this method of measurement cannot be adopted. I 

 may, however, observe that the other method, namely that of commer- 

 cial determination, can scarcely be expected to give more accurate re- 

 sults when the conditions are unfavourable, as for instance on a journey 

 through an extremely wild and dangerous region, where the traveller 

 is obliged to form his own track, and stands every moment in danger 

 of an attack ; under such circumstances all simultaneous observations 

 of the barometer, at the base and summit of mountains, or a series of 

 observations at any one point, are cruite out of the question. Experi- 

 ence has also shewn me the complete impossibility of keeping the 

 barometers (I had two with me) from breaking, in a country so 

 mountainous as that I traversed, where, on each expedition, the pack 

 horses and camels stumbled repeatedly, and were occasionally dashed 

 to pieces by falling over precipices. Hence travellers (Humboldt 

 amongst the rest on his famous journey in the Andes and the 

 Cordilleras) have invariably had recourse to the method of determining 

 heights by the temperature of boiling water. The results obtained in 

 this manner are regarded by science merely as approximations, until 

 they are superseded by more accurate data, obtained when the region 

 is more accessible to scientific exploration. 



Although incomplete, these results are nevertheless of undoubted 

 value to science, as the magnitude of probable errors even under such 

 an imperfect method, cannot exceed certain limits. 



