362 Dr. Carl Barns on the Efficiency of 



dry air or not 



(pj - it) + r/V . (p 2 - tt) = (p s - tt) (1 -f v/Y) 



is constant for a given absorption. 



8p 2 ' = -v/V . o> 2 . 



Hence if hp 2 — 2 cm., since r/V = *064, 



-fyV = -064 x 2 = -13 cm. nearly. 



This case is illustrated graphically for p / = 45 cm. in the 

 notched curves of the figure in a way easily understood. 

 It seems probable that whereas the smaller fog-chamber has 

 lost too much air to even approach the isothermal pressures, 

 p 2 , the large vacuum-chamber is only a millimetre short of 

 them when the cock is again closed. The constancy of the 

 observed difference, p 2 —p s , seemed at first to be referable to 

 the systematic method of investigation, though the effect of 

 the precipitated moisture (which has not yet been considered) 

 will largely account for it. See § 5. 



Anomalous relations in the data for the fog-chamber, as in 

 the case of p' = 59*5 cm., are direct errors of observation. 

 On the other hand, howeA'er 7 since within the ranges of 

 observation and very nearly p = a, p 2 = a 2 -Y b 2 p\ pz = a z -i-b 3 p'\ 

 it follows that {p — P2)l(p-pz) may approximately be written 

 A + Bjt/, where a, b, A, B> &c. are constant. Frequently B 

 is negligible, so that (p2^Pz)/(p~~Pz) * s constant, in which 

 case the graph for p 2 — p z varying with p—p s passes through 

 the origin. 



4. Computation of t±. — To find the temperature of the 

 fog-cbamber after the adiabatic temperature r x has been 

 raised by condensation of fog to t 1? it is apparently necessary 

 to compute^ first, and then proceed by the method used by 

 Wilson * and Thomson. When the vacuum-chamber is 

 large, however, its pressures vary but slightly, and therefore 

 the pressure observed at the vacuum-chamber after exhaustion, 

 p 3 , when the two chambers are in communication, is very 

 nearly the adiabatic pressure of the fog-chamber,^. This 

 result makes it easier to compute not only t 1? but incidentally 

 the water, m, precipitated per cubic centimetre (without 

 stopping to compute the other pressures), with a degree of 

 accuracy more than sufficient when the other measurements 

 depend on the size of coronas. 



To show this, let d, L, and tt refer to the density, latent 



* C. T. E. Wilson : Phil. Trans. London, vol. 189, p. 298 (1897). 



