TERRESTRIAL REFRACTION. 11 



vations, to be 15' 19' 5, and the instrument (in the 

 Mimtapum) being 8, 5 feet above the ground, we 

 have to subtract 1' 6' 59, which reduces the depres- 

 sion to 14' 12"91=J^> a constant quantity in com- 

 puting the third column of refraction. 



Hence we have, 



Z)=14' 12" 91 

 —E:=i\0 06 27 



Tlie refraction entered in the tables. 



The absolute degree of moisture was deduced as 

 follows: 



As I knew of no standard, by which I might set 

 the hygrometer, when I was about observing, the 

 least degree of moisture noticed during the day was 

 assumed as zero. This arrangement had this advan- 

 tage, that the refraction and moisture had a similar 

 direction, and their coincidence met the eye more 

 easily. 



The column which shews the absolute density of 

 the atmosphere was computed by this formula; 



2>=f5~^'>^^'— X 1— ^^* founded on rea- 



4 3 5 



* Where D expresses the increase of density, B the height of 

 barometer at the time of observation, B' the slate of the same when 

 lowest of all, n the difference of temperature in the air, and d the 

 difference of temperature in the mercury. 



