AQUEOUS VAPOUR IN THE POLAR ATMOSPHERE. 357 



These icy particles, if we might venture to hazard 

 an opinion on the subject, proceed from upper currents 

 of air, passing in the great altitudes, still carrying with 

 them some slight quantities of watery vapour, which 

 the exceeding keenness of the arctic cold causes to 

 become frozen, and to be precipitated upon the earth in 

 this way, in the form of very minute ice crystals. The 

 rapid and progressive increase of cold, as the altitude 

 increases, is well known, and throughout the great 

 regions of illimitable space it represents a figure of 

 which we, the dwellers upon the earth, can form not 

 even the faintest conception even from our lowest re- 

 corded temperatures. Dr. Croll for instance mentions 

 in his remarkable work upon " Climate and Time in 

 their Geological Relations, " that " the Temperature of 

 Space as determined by Sir John Herschel is 239 

 Fahr.," and he adds that " Mons. Pouillet by a different 

 method, arrived at almost the same result." * 



Here moreover we touch upon a technical point of 

 very great importance namely the evident existence 

 of aqueous vapour in intensely cold form in the higher 

 atmosphere of the polar regions. As a rule any consider- 

 able lowering of the temperature is sufficient to convert 

 this watery vapour into either snow or rain. It is 

 then precipitated, and we have several times called 

 attention to the fact that even the passage of air over 

 lofty mountains causes the apparently almost complete 

 condensation of its vapour. The fall of these icy crystals 

 during still weather throughout the polar winter, and 

 the occurrence of heavy snowstorms as far northward 

 as man has been known to penetrate, proves however, 



* Climate and Time in their Geological Relations, by James Croll, 

 LLJX, F.R.S., 1885, p. 323. 



