PREFACE 



No one doubts the considerable influence which changes in baro- 

 metric pressure can exercise on living beings; we are even inclined 

 to exaggerate its importance. If the barometric column rises or 

 falls some millimeters, nervous or asthmatic people experience 

 favorable or painful symptoms which they attribute to the heavi- 

 ness or the lightness of the air. If this were really the cause, a 

 walk from the banks of the Seine to the top of the Butte Mont- 

 martre or the converse should produce similar results in the same 

 people. 



But outside this group of data, to which I shall return in a mo- 

 ment, many remain which present a much greater interest, and 

 which deserve to be studied with perseverance. 



Are we dealing with increase in pressure? When, in the shafts 

 of a mine or in the caissons intended to become the piers of a 

 bridge, workmen are protected against the invasion of the water 

 by air compressed by powerful machines to several atmospheres, 

 they experience strange and sometimes dangerous symptoms dur- 

 ing or after their stay in compressed air. Likewise divers who 

 gather pearls, sponges, or coral, or attempt the salvage of sunken 

 ships, furnished with diving apparatus and breathing an air whose 

 pressure is proportional to the depth they reach, are frequently 

 stricken by paralysis or death. On the other hand, medicine, mak- 

 ing use of observations that are already old, has attempted with 

 considerable success to make use of the influence of air at suitably 

 low pressures, since the time of Junod, Pravaz, and Tabarie. 



Are we dealing with decrease in pressure? We can mention 

 first the symptoms which threaten aeronauts when their ascent 

 brings them to heights above 4000 meters: nausea, vertigo, hemor- 

 rhage, syncope; then the phenomena which have been known much 

 longer by all those who have attempted the ascent of mountains 

 of over 3000 to 4000 meters, mountain sickness, about whose cause 



XV 



