318 Historical 



and thence came also the belief in metallic emanations, mephitic 

 gases issuing from the ground, and fatal exhalations from different 

 plants. 



But, barring these very interesting exceptions which we shall 

 try to explain in another part of this work, the differences in aver- 

 age height at which serious symptoms appear according to the 

 parts of the world in which they are observed are in a remarkable 

 agreement with differences in the altitude of the line of perpetual 

 snow, when we consider them as a whole. The summary which 

 we inserted earlier (see page 16) on this latter subject facilitates 

 this comparison for the reader. But we must not go so far as to 

 believe, as some travellers have done, that a direct relation, 

 almost of cause and effect, exists between these two distinct 

 orders of phenomena. Very evidently, no one has ever complained 

 of mountain sickness in the polar regions, where the lowest hills 

 are eternally covered with snow. But without having recourse to 

 this reductio ad absurdum, we see that in our Alps it is almost 

 always 500 meters at least above the line of melting where physi- 

 ological disturbances appear with sufficient intensity to attract 

 attention. The same thing is true upon the volcanoes of Ecuador 

 and Mexico, the Rocky Mountains, and many other points. On the 

 contrary, on the Bolivian Andes and still more on the Himalayas, 

 the narratives previously published show us that travellers may 

 be very sick when they are treading terra firma, and are still quite 

 far from the zone of perpetual snow. But it is no less true to say 

 that, in a general way, the higher the line of perpetual snow, the 

 later will travellers in their ascent be threatened with the symp- 

 toms which we have so often described. 



Besides these irregularities due to exterior circumstances, there 

 are some which depend upon the idiosyncracies of the travellers 

 who are subjected to the effect of decompression. 



Indeed, in the same region, on the same mountain, we see 

 travellers sometimes complaining of severe sufferings, sometimes 

 rejoicing or expressing surprise at having felt almost no distress. 

 On the pass of Cumbre of Uspallata, most of those who are crossing 

 the Andes are attacked by the puna; Samuel Haigh, Schmidtmeyer, 

 and many others have testified to it: but we have seen that Miers, 

 Brand, Strobel, etc. escaped it entirely. Whereas von Humboldt 

 and Bonpland were very sick in their ascents of Chimborazo, M. 

 Boussingault and Colonel Hall, who ascended higher than they, 

 experienced only slight symptoms, and M. Jules Remy, who says 

 that he reached the summit, states that he felt no symptom of ill- 

 ness, On Popocatepetl, Bacon Gros and his six companions, and 



