400 SUDDEN CHANGES OF TEMPERATURE. 



to the careful student, who not unfrequently finds 

 himself unwittingly led astray by them. The existence 

 of these cold winds in the Desert Zone, so far as 

 we are aware, has long" been a matter of well ascertained 

 fact, and the comparatively limited area of the desert 

 in northern Mexico and Texas, may be cited as an 

 example of a region peculiarly subject to the visitation of 

 these " cold snaps " as they are there termed. Similar 

 experiences are noted from the great Central Asian 

 deserts, as indeed from all other dry barren regions. 



There is hardly much room for doubt that the cause 

 of the sudden transitions from heat to cold, in the 

 case of these cold winds, is the displacement of a warm 

 undercurrent by a cold layer of air descending from 

 the higher regions of the atmosphere, as we have had 

 occasion to point out elsewhere while the rapid chilling 

 of the still air by night is due to radiation and 

 absence of aqueous vapour. Count D'Escayrac de 

 Lauture mentions the fact of his observing on one 

 occasion, during the month of January, under the 17 th 

 parallel of latitude, the thermometer which at sunrise 

 marked a temperature of only 5 Celsius (= 4 1 Fahr.) 

 rising to 35 Celsius (= 95 Fahr.) by i. p.m. the 

 following afternoon, thus showing the enormous varia- 

 tion of 30 Celsius (=54 F.) in only seven hours. 

 The human frame, he says, 



" has not time to accustom itself to these sudden changes, 

 the cold of the night seems insupportable and the heat of 

 the day overpowering; these two temperatures are nothing 

 excessive in themselves, but the human body, able to bear 

 the winters of the Polar regions, and the summers of the 

 Soudan, does not bear equally well the sudden transition 

 from the temperature of a night in France to that of a day 



