so many strange hypotheses have been suggested. Finally we 

 find here data of a much greater importance. It is no longer a 

 matter of a few workmen, a few invalids, or a few tourists, but of 

 whole populations which normally and regularly live, construct 

 cities, group themselves as peoples, in these lofty places where 

 painful and sometimes unendurable sensations await the traveler. 



We feel that here our problem affects not only the hygiene of 

 peoples, but also to a certain point, their history and politics. In 

 the Himalayas, in the Cordillera of the Andes, populous cities are 

 built at heights greater than that of our Mont-Blanc, where no one 

 completely escapes mountain sickness; in Mexico, thousands of 

 men live on the plateaux of Anahuac, at an average height of 

 2000 meters; the great civilizations of the Mayas and the Nahuas 

 had their maximum of development between 2000 and 4000 meters 

 above sea level. 



The reader can see by this brief survey in what important 

 points the question affects the experimentation to which I have 

 conscientiously devoted myself. It will consequently seem natural 

 that such phenomena have given rise to numerous publications by 

 doctors or travelers; but he will no doubt be surprised that so little 

 has been attempted in laboratory experimentation to explain their 

 cause. The simplest idea apparently would have been to construct 

 apparatuses permitting one to reproduce changes in barometric 

 pressure, isolating them from secondary conditions, uncontrolled 

 variables, which inevitably accompany them in the state of nature, 

 and to examine the immediate results of these changes on man and 

 on animals. Now very little has been done in this direction. On 

 the other hand, we shall find incomplete observations, pretentious 

 dissertations, and probable or absurd explanations in great number. 



My purpose has been to fill this considerable gap, and to solve 

 these important problems by a purely experimental method. 



In taking my position thus on solid ground, I had to set 

 aside systematically three kinds of questions which could not be 

 attacked in the laboratory, and for which consequently absolute 

 conditions of proof could not be collected; that is: daily variations 

 of the barometer, therapeutic applications and acclimatization in 

 lofty places. 



I do not regret the first question, which does not seem to me 

 even to belong to our subject of study. Slight modifications in air 

 pressure revealed by the barometric column in a given place are 

 accompanied by too many other meteorological phenomena (hygro- 

 metric, electric, etc.) for anyone to determine the part, certainly 

 very small, which they play in the condition of certain invalids. 



XVI 



