800 ATMOSPHERICAL PRESSURE 



or 40 feet deep into water in a diving bell, 

 the pressure of the air upon the body is in- 

 creased inwards ; pains in the ears are felt from 

 the difficulty of suddenly restoring a disturbed 

 equilibrium; but if the descent is slow and inter- 

 rupted, time is given for the air to enter the pores, 

 and the pain is less sensible. To what limit 

 warm blooded animals could bear the rarefaction 

 of air so as to subsist, has not, that I am aware 

 of, been determined with much precision. 

 Ascents in balloons have been made till the 

 atmospheric pressure was reduced more than one 

 half. Formerly I found that a mouse could 

 subsist in air of J of atmospheric density, and 

 seemed not to have suffered much; but upon 

 reducing the density below J, the animal was 

 convulsed and expired immediately, notwith- 

 standing the air was instantly admitted. 



If the view we have expanded in this e^say, in 

 regard to the action of aerial pressure on the animal 

 frame, be correct, it may be inferred, that the pres- 

 sure admits of great latitude ; perhaps an animal 

 could subsist under the pressure of i an atmosphere, 

 or of 3 or 4, or more atmospheres. The uneasiness 

 and danger would be found in the quick transition. 

 If time is allowed for the air to enter the body, 

 and to escape from it, the transition is gradual, 

 and the sensation arising from it imperceptible. 



