[ "4 ] 



(tliat is, the iinaginary uniform temperature) be nearly 32*; 

 otlierwife call this refult the approximate height. Multiply the 

 approximate height by 6 to reduce it to feet. 



RULE IV. 



Seek in the table, for corredlion of rarefradion of the atmo- 

 fphere, the numbers anfwering to the numbers of feet in the 

 approximate height. Multiply the fum of thefe numbers by 

 the difference in degrees and decimals of a degree between the 

 imaginary uniform temperature and 32°. If the imaginary 

 unform temperature exceed 32°. add this produ6l to the ap- 

 proximate height, and the fum will be the adual difference 

 of the heights of the flations in Englifli feet. This method 

 of inveftigation is deduced from a paper of Dr. Mafkelyne's, 

 founded on the calculation of Mr. De Luc and Sir George 

 Shuckburgh; and to make it more pradlicable and lefs 

 complicated the algebraic demonftrations are omitted. If 

 General Roy's calciilations of the effeds of heat and cold 

 on the atmofphere and on the mercury be more accurate than 

 any former ones, they may be eafily adapted to thefe formiile^ 

 and tables calculated from them. 



POSTSCRIPT. 



This communication on the fubjed of the portable barometer 

 was fome time fince fubmitted to the confideration of the learned 



and 



