PHYSIOLOGY OF THE CIRCULATION. 113 



thereby to descend at least two inches. Nay, that 

 barometer which was three feet distant from the 

 aerial stream was equally affected as the nearer one. 

 And it is further observable, that as the force of the 

 current of air diminished, so the weight of the 

 atmosphere recovered its strength again, and forced 

 the mercury in the barometers to a gradual ascent." 

 This experiment was undertaken with the view of 

 discovering the cause of the descent of the mercury 

 in barometers during a storm. And I may add, that 

 the few remarks which the author makes upon the 

 probable influence on the animal body of a diminished 

 atmospheric pressure, are, so far as they extend, 

 very much in unison with the physiological prin- 

 ciples advocated in this communication. 



It is therefore evident, that the entrance of an 

 external fluid into the interior of a porous tube tra- 

 versed by a rapid stream, is a consequence of the 

 pressure then acting on the internal surface of that 

 tube being less than that of the surrounding 

 medium. 



When the tube is exposed to the air, and a quan- 

 tity of this gaseous fluid is absorbed, the pressure of 

 the atmosphere is evidently the sole cause of the 

 phenomenon; but when liquids are absorbed, the 

 weight of the external stagnant column co-operates 

 with that of the atmosphere in effecting absorption. 



In the animal body, this external pressure the 

 immediate cause of absorption - - varies much in 



: " Physico- Mechanical Experiments," pp. 114, 115. By F. 

 llauksbee, F.R.S. Second edition. London, 1719. 



I 



