the AtmospJierkal Pressure on the Animal Frame, 91 



has to sustain this pressure ; and it will be found by calculation, 

 that the whole surface of a middle-sized person, will have to sup- 

 port from 15 to 20 tons of pressure all acting inwards, and hav- 

 ing no other mechanical tendency than that of squeezing or com- 

 pressing the materials of which the body is composed into a less 

 compass. 



The above is a statement of facts, all of which I believe to be 

 incontrovertible. But a very difficult question arises out of them. 

 How is it that the animal frame is utterly insensible of the whole, 

 or of any part of this enormous pressure upon it ? In ordinary 

 we feel no pressure on the surface of our bodies, either external 

 or internal ; neither when the barometer is stationary nor when 

 it is in a most fluctuating state. I have never met with a satis- 

 factory answer to this question, and I doubt whether such a one 

 has ever been given ; yet it must be allowed to be one of import- 

 ance, both as it affects the physiology of the animal and vegeta- 

 ble kingdoms. Having had occasion for a few weeks past to 

 ruminate on this subject, some new views have occurred to me ; 

 and it is the object of the present essay to unfold them, in 

 order to elucidate the phenomena arising from aerial pressure on 

 the animal economy more especially. 



It is pretty well known that the specific gravity of living men 

 in general, is less than that of water. Mr Robertson, formerly 

 librarian to the Royal Society, procured an apparatus for the 

 purpose of determining the specific gravity of the human body. 

 He chose ten men promiscuously for the purpose. Of these, 

 three were found very nearly of the same weight as water, one 

 being a little heavier, and the other two a little lighter than wa- 

 ter ; two others were found only about ,8 the weight of water ; 

 but the other five were of intermediate specific gravities. The 

 average of the ten was, height 5 feet 6| inches; weight, 146 lbs. ; 

 specific gravity, 891 ; bulk, 2.61 8 cubic feet. From this I think 

 we may safely infer that the body of a full grown living man, 

 when plunged over head in water, will be found upon the average 

 to be nearly .9, the weight of an equal bulk of water. 



It is remarkable that all the component parts of the animal 

 frame, at least of the human subject, are severally specifically 

 heavier than the whole body, with the exception of air. Bone, 

 muscular flesh, blood, membrane, &c. are all heavier than water ; 



