foodies, with awei'ght equal to a pillar of air, whose base 

 is equal to the surface of our bodies. Now a pillar of 

 air of the height of the atmosphere, is equal to a pillar 

 of water thirty-two feet high. Every foot square there- 

 fore of the surface of our bodies is pressed on by a 

 weight of air, equal to 35 cubic feet of water : and a 

 cubic foot of water weighing ?6 pound, (Troy weight;, 

 consequently every foot square of the surface of our 

 bodies sustain a pillar of air, equal to 2,S()0 pounds. If 

 then (he surface of a man's body contains fifteen square 

 feet, he sustains a weight equal to 3 t 9,9pO pounds. This 

 is the case, when the air is heaviest. But the difference 

 between the greatest and the least pressure of air upon 

 our bodies is equal to 3982 pounds. 



Hence it is so far from being a wonder, we should 

 sometimes suffer in our health, by a change of weather, 

 that it is the greatest wonder we should not always suffer. 

 For when we consider our bodies are at sometimes prest 

 upon by near two tons weight more than at others, it is 

 surprising that every change does not break our whole 

 frame to pieces. 



In truth the vessels of our bodies being so much strait- 

 ened by an increased pressure, would stagnate the blood 

 to the very heart, had not the Author of nature \visely 

 contrived, that when the resistance to its circulation is 

 greatest, the force by which the heat contracts should be 

 so too. For upon an increase of the weight of the air, 

 the lungs are more strongly expanded, and the blood by 

 being more intimately broken, made fitter for finer secre- 

 tions; the nervous in particular, by which the heart is 

 more strongly contracted. On the other hand, when 

 the weight tff the ambient air is ever so little abated, the 

 air contained within the blood, unfolds its springs, and 

 forces the blood to take up a largtr space than it did 

 before. 



The reason we are not sensible of this pressure, is well 

 explained by Borelli : sand, perfectly rammed into a hard 

 vessel, cannot be penetrated, even by a wedge. And 

 water m a bladder, compressed on all sides, cannot give 

 nay in any part. In like manner, within the skin of an 



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