100 Mr H. Meikle onjinding the Dezv-Point, S^c, 



D-v^--0^ t 



and therefore pn^ = , ^ ^ 



[CF + */PN» + b^Y — PH« 



Hence, because pw, as already mentioned, is a mean propor- 

 tional between EI and PH, we have 



EI - ^' = ^^PH 



PH [CF + VPN2 + 62P — PH2. 



• Since a has entirely disappeared, this result is independent of 

 the value of the transverse axis, which may therefore be taken 

 of any magnitude most convenient ; but, in order to apply this 

 general expression, we must first, supposing EI to be known, 

 find, by means of the same equation, the value of 6', which, 

 after going through the necessary steps and reductions, will be 

 4PH XEI XCF _ /, CF 



PH_EI 



/ CF \ 



^ V "*'PH — EI/. 



From examining the large Table, page 284 of this Journal 

 for September last, it appears that when t\ the temperature of 

 the wet thermometer, is 54° F, it is always nearly the arithme- 

 tical mean between t the temperature of the air, and f the dew- 

 point. ,Now, when the straight line or ruler PE, in the graphic 

 scheme, passes through the centre C of the hyperbolas, we have 

 always PH — EI, or f equal the arithmetical mean between t 

 and f. Hence, the transverse axis cuts this scheme in about 

 the 54° of Fahrenheit; and therefore CFrrjJ' — 54°; PN^ 

 < — 54°. Also, when t = 56% and f z= 45°, we find f = 26° in 

 the Table; and consequently CF=r9; EI = 19 ; PH=rll, 

 which values being substituted for these quantities respectively 

 • in the general expression for 5^, it becomes 



and b = 44°.7. Again, when t = 92°, and f = 70°, we find 



f = 6l°-2in the Table; 



--. ,, 4x22x8.8x16 /, . 16 \ „^^^ ^ 

 Hence b^ = ^^ x (l + -^3^) = 2076.5, 



and 6=45°.57. It thus appears, that the extremes of the 

 range, as yet compared with observation, would nearly agree in 

 making 6 = 45°; so that the conjugate axis corresponds to an 



