AND ANIMAL LIFE. 421 



undergoes on the removal of pressure, when the 

 body is completely at rest, or when the whole 

 system is animated by exercise, have differed 

 among themselves as to the precise operation of 

 this power. When an individual ascends one of 

 the loftiest mountains, he has great difficulty of 

 breathing. The countenance is extremely flush- 

 ed, and the veins on the surface of the body are 

 much swollen. These and a variety of other symp- 

 toms are said to be the results of diminished 

 pressure ; but when the aeronaut is elevated to 

 an equal height, he does not experience the same 

 painful sensations, nor does the system exhibit 

 the same effects. The evident difference in the 

 consequences, when the body is similarly exposed 

 to the influence of the same agent, is a circum- 

 stance which militates materially against the ra- 

 tionale proposed by the most enlightened physi- 

 cians. 



DXIV. The nature of these physiological 

 studies have naturally led me to investigate the 

 influence of pressure, when exerted alone, and 

 when in combination with exercise ; and the 

 principles deduced appear to harmonize the di- 

 versities of opinion previously entertained on 

 this subject. When the body is in a state of re- 

 pose, and subjected to a cold atmosphere, which 

 is always the case with the aeronaut, the flow of 

 the blood becomes more internal than external, 

 in consequence of which the diminished pressure 

 has to act upon a less quantity of sanguineous fluid, 



