88 CAUSES which affeS the ACC URAC Y 



fubtracted for every degree of the thermometer. By obferva- 

 tions ftill more accurate, it has been found, that the tempera- 

 ture at which the difference of the logarithms gives the height 

 in Englifh fathoms, is 32 ; and that the correction at other 

 temperatures is .00243 of that difference, for every degree of 

 the thermometer *. The manner of eflimating the tempera- 

 ture of the air, adopted in all thefe obfervations, was the fame j 

 an arithmetical mean was taken between the heights of the 

 thermometers, at the upper and lower ftations, and was fup- 

 pofed to be uniformly diffufed through the column of air in- 

 tercepted between them. M. DE Luc, however, was fenfible 

 that this fuppofition was inaccurate ; and General ROY, too, has 

 obferved, that " one of the chief caufes of error in barome- 

 ' trical computations proceeds from the mode of eftimating 

 " the temperature of the column of air from that of its extre- 

 ' mities, which muft be faulty in proportion as the height and 

 ' difference of temperature are great f." It will appear, how- 

 ever, that this eftimation, though adopted merely on account 

 of its Simplicity, and probably on no other principle than the 

 general one of taking a mean between two obfervations, which, 

 taken fingly, are inaccurate, comes nearer to the truth than 

 there was any reafon to expect. 



2. IT is certain, that the atmofphere does not derive its heat 

 from the immediate action of the folar rays. Thefe rays, in 

 traverfing that fubtle and tranfparent medium, are but (light- 

 ly refracted, and, meeting with little obftruction, neither lofc 

 nor communicate much of their influence. We are aflured of 

 this by many experiments ; and we know, that air, in the fodus 

 of a burning glafs, is never heated till fome folid body be in- 

 troduced. 



* General ROY makes the fixed temperature 32, and the expanfion for i, = .00245, 

 at a medium. Sir G. SHUCKBURGH makes the fixed temperature 31*^, and the expanfion, 

 as here afllgned, viz. .00243. Pi>i/. Tranf, 1777. It is fufficient for us at prefent to know 

 thefe numbers nearly. According to the formula laid down hereafter, they will all re- 

 quire to be corrected. 



+ rial. Tranf. 1777. 



