On Atmospheric Density and Pressure. 423 



for 850 or 900 feet at a medium* : it' this ratio held good 

 altogether, the mercury must remain in its bed at the height 

 of 27000 feet. 



As a square inch column of mercury 30 inches higlr 

 (which various observations and experiments have proved to 

 be the medium pressure in those countries, or others situated 

 in or near their latitudes,) weighs about ljlbs., so the at- 

 mosphere must press with a weight equal to 15lbs. on every 

 square inch, or 180ibs. on each square foot: calculating thus, 

 the whole superincumbent pressure of the atmosphere 

 amounts to 12,043,44 8,800,000, 000,000lbs.f Reckoning 

 the surface of a man's body to be about 14 square feet, he 

 sustains a pressure (calculating thus) of 1 1 tons 2 hundred 

 weight 18}lbs. 



It mav be wondered how man could bear such a weight 

 on his body and not be crushed. On the contrary, this pres- 

 sure is indispensably necessary to our existence, and two 

 reasons concur in preventing its being felt troublesome : 1st, 

 its being so equal all over our bodies as not to move their 

 fibres; 2dly, the caloric generally evolving from hence 

 counteracts and renders it less sensible. Besides, sensations 

 we have been accustomed to from our birth do not much 

 annoy us, and it is probably this pressure that occasions the 

 cries of the new-born babe. 



Atmospheric pressure is necessary to our existence. In 

 ascending into the air, we might suppose our sensations 

 would be more agreeable by being loosed as it were from a 

 heavy load : yet the contrary is the fact ; the blood of our 

 internal vessels not being pressed down, bursts and over- 

 flows its barriers, insomuch as sometimes to endanger death. 



These sensations are always perceived in ascending con- 

 siderable heights : spitting of blood, and bleeding at the nose 

 and eyes, a drowsiness, listless apathy, and inexertion are 



* This appears from the observations and experiments of Saussure and 

 De Luc on the Alps, Don Ulloa, i'ouguer and Condamine on the Andes, 

 and of Mr. Kirvvan on mountains in Ireland ; all of whose labours "in this 

 interesting subject, besides those of Dr. Hailey, Sir George Shuckburgh, and 

 General Roy, have so much contributed to its advancement. 



f Equal to a ball of lead 60 miles in diameter. See Coles's Lectures. 



D d 4 expe- 



