354 Historical 



pressures which are sometimes enormous; attention has been at- 

 tracted only to symptoms appearing when these pressures were 

 very high; and finally, these symptoms are, as we shall see, the 

 result, not of the compression itself, but of sudden decompression. 

 In medical apparatuses, on the contrary, the pressure used has 

 always been low, less than double normal pressure; the physi- 

 ological observations have been made on the phenomena produced 

 by the compressed air itself, and no symptom could be charged 

 to the decompression, which was always very gradual. 



I therefore had to divide into two chapters the report of such 

 different data. A third is devoted to the account of the attempts 

 made by various authors to explain the physiological changes and 

 the more or less dangerous symptoms which attack the workmen. 

 Finally, in the last chapter, I have summarized and discussed the 

 data observed and the theories suggested, with the purpose of 

 explaining them. 



1 The small, almost uninhabited regions, where the ground is below sea-leve 

 course, be excepted; the most important, certainly, is the valley of the Dead Sea. 



