42i On Atmospheric Density and Pressure. 



^experienced*. Hence Providence has wisely ordered it, that 

 this pressure, though apparently such a load on us, should 

 be absolutely necessary to our lives, by keeping our blood 

 within its proper limits, making its particles retain their 

 proper cohesion, and thus giving the pulse, heart, &c, that 

 uniform gradation of beating in which the vital principle 

 consists. 



Atmospheric pressure varies considerably, (as was before 

 observed,) particularly in the upper regions, and these varia- 

 tions appear to increase as we ascend into them ; because the 

 decrease of density not keeping pace with the decrease 

 of pressure, there must, in those high regions, be sudden 

 condensations and rarefactions, which must divert from their 

 direction the pressing down particles of the atmosphere, and 

 drive them laterally and otherwise, &c. Hence the secon- 

 dary cause that affects these variations is the winds : 011 this 

 account these variations are greatest at the poles, and de- 

 crease towards the equator, there being greater winds at 

 the former, the atmosphere there being denser from its less 

 high state : that they are little or none at the equator, has 

 often been proved. 



Were it not for atmospheric pressure there would be but 

 little distinction between liquids and gases, no more than 

 what is produced by the action of that aggregate attraction 

 on the one hand, and that quantity of caloric on the other, 

 with which they are naturally endued. For, as the pressure 

 always assists cohesion, and consequently counteracts the 

 effect of caloric ; and as the particles of bodies have liberty 

 to move and recede from each other directly as the positive 

 and repelling power, and inversely as the negative and ap- 

 proximating one, viz., cohesion ; and as between these con- 

 stituent internal powers all bodies are balanced ; it follows, 

 that when a third neutral external power is introduced, viz., 

 pressure, it must turn the scale, weaken the action of the 

 caloric, and tend to keep the body in a liquid state ac- 

 cording to its force. Hence the quantity or degree of heat 



* Messrs. Humboldt and Bonpland perceived these symptoms to a verv 

 alarming degrcee on the Cordilleras : even Dr. Pitcairn in ascending Ar- 

 thur's Seat (only 900 feet) began to be sensibly affected so. 



necessary 



