the Atmospherical Pressure on the Animal Frame. 9^ 



30 or 40 feet deep into the water in a diving bell, the pressure 

 of the air upon the body is increased inwards ; pains in the ears 

 are felt from the difficulty of suddenly restoring a disturbed 

 equilibrium ; but if the descent is slow and interrupted, time is 

 given for the air to enter the pores, and the pain is less sensible. 

 To what limit warm-blooded animals could bear rarefaction of 

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

 ed 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 ^th of atmo- 

 spheric density and seemed not to have suffered much ; but upon 

 reducing the density below ^th, the animal was convulsed and 

 expired immediately, notwithstanding the air was instantly ad- 

 mitted. 



If the view we have expounded in this essay, in regard to 

 the action of aerial pressure on the animal frame, be correct, 

 it may be inferred, that the pressure admits of great latitude ; 

 perhaps an animal could subsist under the pressure of half an at- 

 mosphere, or of three or four, or more atmospheres. The uneasi- 

 ness 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 imper- 

 ceptible. The animal economy would be adapted to it, like as 

 in the transition from a cold to a warm climate. It may here*, 

 after be found, what length of time is sufficient to adjust the 

 equilibrium, and whether this subject is any way connected with 

 certain diseased states of the body. As far as regards the ab- 

 solute pressure on the body, and our insensibility of it generally, 

 this question will be met by the argument, that the air within 

 the body, by its elasticity, sustains a corresponding pressure 

 from without ; but this only accounts for our alleviation from a 

 small fractional part of the whole exterior pressure. The great- 

 er part must still be supported by the body ; and we must have 

 recourse to the great incompressibility of matter to account for 

 our insensibility of pressure. Canton found that water, pressed 

 by one atmosphere more than ordinary, only exhibited a reduc^ 

 tion of 2i7 4c^h P^*"^ o^ the whole ; if the same rate, applied to 

 the compression of the human body, the reduction or compres- 

 sion of the size of a man, 4500 cubic inches, would only be Jth 



