BAROMETER TO SEA LEVEL. 



IFrom (iv.) and (v.), together with (vi.), we may deduce (ii.), thus ; 

 In (iv.), let & = /3, we obtain 



_Z_ \_ _1_ _Z_ ^ 1 



' fi' '""* '-''A 'a" "'^^ 





Similarly from (v.) making b = B, 



5 f 10 L2 ' 10 ■ ^N^ 10 B ■*■ • 



jO t 



A+„^.U=1fH-4r-|- U^-.:J-^+^„+ :' 



^ ^ t] 10 1.2 10 1 10/3 oiV^j; 105. iVr ^10/3"^ 1051 +• 

 But from (vi.) B. N = B. ^JV nearly. 



,% \&^t "^ 5^i) ^ - 10 + T2"* lo' (io7 + iol ^®^^^:^' 



or, neglecting the second term on the right, 



^ t ^ B t 



Here t is the mean between the temperatures at the upper and 

 lower stations; whilst in (ii) these two temperatures are respec- 

 tively employed, in taking out the two numbers. The difference 

 thus introduced is very trifling ; as may easily be seen, if the value 

 given below for A , be substituted in the expression for N 



Formula (ii.), like (v.), is objectionable, in that it assumes a know- 

 ledge of the reduced reading, which it is the object to ascerta,in. 



