394 JUDGING DISTANCES IN THE DESERT. 



from our consideration, it is, we think, desirable to 

 remind the reader how greatly the extreme transpar- 

 ency of the atmosphere, the spacious grandeur, uni- 

 formity and emptiness, of these boundless plains, tend 

 to produce various kinds of optical illusions. 



This is very forcibly brought home to the mind 

 when attempting to estimate the distances, or dimensions, 

 of objects seen in the landscape. There is generally 

 nothing to guide the eye, and for want of any standard 

 of comparison, everything appears to be much nearer 

 than it is in reality. All guesses respecting distance, 

 etc., are therefore sure to prove very wide of the mark. 

 Thus mountains which, at sunrise, stand out with 

 extreme distinctness against the sky line, at apparently 

 no very great distance away, will often actually seem 

 to recede before the traveller, throughout the day's 

 march, and not uncommonly disappear altogether during 

 the afternoon, in the quivering haze refracted from the 

 heated ground, but only to reappear again the next morn- 

 ing, as if they had again advanced to meet the observer. 



This phenomenon which is probably familiar to most 

 travellers who have had much experience in plains- 

 travel, is peculiarly noticeable upon the extensive table 

 lands lying near the base of the Andes in South 

 America, where the Cordilleras often appear to rise 

 out of the plain a short distance ahead, while in point 

 of fact they may be 60 or 70 miles away. 



In the peculiarly clear atmosphere existing upon dry 

 elevated plains, mountains are frequently visible at 

 distances which would be thought incredible in damp 

 tropical or temperate climates, where the horizon is 

 always more or less misty. 



From the decks of steamers proceeding along the 



