Evaporation from a Circular Water Surface. 315 



In Brown and Escombe's well-known paper on diffusion * 

 there is in places an obscurity of terminology which renders 

 the account needlessly difficult to follow. Thus (p. 228, 

 note) we read of a density gradient of one atmosphere 

 per metre ; on p. 251 we have Stefan's formula wrongly 

 approximated, with the substitution of "mass" for "volume*" 

 already discussed in connexion with the same mistake in 

 Preston's ' Heat ' ; on the same page we have an equation 

 for the rate of absorption (Q) of atmospheric C0 2 by an 

 absorbing disk of diameter D, viz., 



Q=2k P D, 



where p is the density of atmospheric C0 2 . Q should there- 

 fore, as dimensional considerations show, be given in grams 

 per unit time, whilst in the experimental discussion of the 

 formula the amounts absorbed are given in cubic centimetres 

 per hour. 



Again (p. 263, note), the volume (Qv ) of C0 2 diffusing 

 per hour down a tube of cross-section A is given at 0° and 

 760 mm. by 



Q v - Ml to _P_ y o Cm 



where L + # is the "corrected" length of the tube, k the 

 coefficient of diffusion, and p the density of the C0 2 in the 

 atmosphere. This formula again is dimensionally impossible, 

 as the right-hand side has dimensions MT" 1 , whilst those of 

 the left-hand side are L 3 T _1 . Tt is on turning to p. 232, 

 where we find density and partial pressure treated as con- 

 vertible terms, that the reason for the discrepancy begins to 

 be apparent ; and on p. 239 the matter is cleared up. Here 

 the symbols used in the equation just mentioned are defined, 

 and p u the density of the carbon dioxide in the atmosphere, 

 is defined as the volume of C0 2 contained in unit volume of 

 air. This arbitrary use of the word density also explains the 

 apparent anomaly in the handling of the equation 



Q, = 2kpD, 



but it appears to us to be a quite indefensible use of so well 

 defined a term, and one which can only make for confusion. 



We proceed now to give a brief account of some experi- 

 ments which we have made on the evaporation from circular 



* Phil. Trans., B, p. 223 (1900). 



