III. On the C A u s E s which affeft A 



TRICAL MEASUREMENTS. By JOHN PLATFAIR^ A. M. 

 F. R. S. EDIN. and Prof ej/br of Mathematics in the Univerfity 

 o EDINBURGH. 



\Readby the Author, March I. 1784. and Jan. 10. 1785.] 



'HP HOUGH the labours of M. DE Luc, and of the excellent 

 * obfervers who followed him, have brought the barome- 

 trical meafurement of heights to very great exactnefs, they 

 have not yet given to it the utmoft perfection it can attain. 

 Some caufes of inaccuracy are ftill involved in it ; of which 

 we ought, at lead, to eftimate the effects, if we cannot correct 

 them altogether. The allowance made on account of the tem- 

 perature of the air, implies in it a hypothefis that has not been 

 examined, nor even exprefled ; and many other circumftances 

 that affect the denfity of the atmofphere, have either been whol- 

 ly omitted, or improperly introduced. The object of the pre- 

 fent paper is to correct the errors that arife from thefe caufes, 

 or, where that cannot be done, to affign the limits within which 

 thofe errors are contained. 



i. THE mpft important correction introduced by M. DE Luc, 

 is that which depends on the temperature of the air. His ob- 

 fepvations led him to conclude, that, at a certain temperature, 

 marked nearly by 69^ of FAHRENHEIT, the difference of 

 the logarithms of the heights of the mercury in the barome- 

 ter, at the upper and the lower flations, gave the height of the 

 former of thofe ftations above the latter in i oooths of a French 

 toife ; but that at every other temperature above or below 

 69 |, a correction of .00223 of the whole was to be added or 



fubtracted 



