3oa 



PHILOSOPHICAL TRANSACTIONS. 



[anno 1686. 



tabular logarithms, being 0,0144765, it will be as 0,0144765 to the difference 

 of the logarithms of 30 and any other lesser number, so 900 feet, or the space 

 answering to an inch of mercury, if the air were equally pressed with 30 inches 

 of mercury, and every where alike, to the height of the barometer in the air, 

 where it will stand at that lesser number of inches : and by the converse of 

 this proportion may the height of the mercury be found, having the altitude of 

 the place given. From these rules I derived the following tables. 



ji table showing the altitude to 

 given heights of the mercury. 



Feet. 



O 



915 



1 862 



2844 



3863 



4922 



10947 



18715 



29662 



48378 



91831 



Inch. 



30 



29 



28 



27 



26 



25 



20 



15 



10 



6 



1 



0,5 110547 



0,25 129262 



0,1 29 mil. or 154000 



0,01 41 mil. 216169 



0,001 •. 53 mil. 278338 



^ table showing the heights of 

 the mercury at given altitudes. 



Feet. Inch. 



30.00 



1000 28.91 



2000 27,86 



3000 26.85 



4000 25.87 



5000 24.93 



. 1 mile 24.67 



2 20.29 



3 16.68 



4 13.72 



5 1 1 .28 



10 4.24 



15 1,60 



20 0.95 



25 0.23 



30 0.08 



40 0.012 



On these suppositions it appears, that at the height of 41 miles, the air is 

 so rarefied as to take up 3000 times the space it occupies here at the earth's 

 surface, and at 53 miles high, it would be expanded above 30000 times ; but it 

 is probable that the utmost power of its spring cannot exert itself to so great an 

 extension, and that no part of the atmosphere reaches above 45 miles from the 

 surface of the earth. 



This seems confirmed from the observations of the crepusculum, which is 

 observed commonly to begin and end when the sun is about 18 degrees below 

 the horizon ; for supposing the air to reflect light from its most rarefied parts, 

 and that as long as the sun illuminates any of its atoms, they are visible to an 

 eye not intercepted by the curvity of the earth ; it will follow, from fig. 8, that 

 the. proportion oi' the height of the whole air, to the semidiameter of the earth. 



