534 



MR A. J. HERBERTSON ON 



Assuming these lines to be parallel and equidistant, which they approximately are, 

 their equatiou is (when t is the temperature of the dry-bulb thermometer in centigrade 

 degrees, t — if the difference between it and that of the wet-bulb one, and w is the 

 weight of water vapour per litre of air expressed in milligrammes) 



t - t' = \ (t + 20) - \-&w. 



Comparing Glaisher's and Guyot's lines for 1'31 grain per cubic foot together, 

 it is seen that from 20°'3 F. to over 31° F. Glaisher gives a smaller difference 

 between dry and wet readings for the same quantity of water vapour, and that at 

 a temperature below 36° F., and for all above it, the conditions are reversed, 

 and Guyot needs a smaller difference ; and lastly, that at about 42° F. Glaisher's 

 curve for 1*31 cuts Guyot's for 0*87. Otherwise expressed, this may be summarised 

 by saying that at temperatures below the freezing-point Glaisher gives much lower 

 dew-points (i.e. his factors are too high), and above 35° F. (l 0- 7 C.) his dew-points are 

 higher (i.e. his factors are too low), compared with those derived from Regnault's 

 formulae, as well as with those obtained by Dr H. N. Dickson ; * and further, that the 

 present experiments show analogous results. The differences at the higher temperatures 

 are by no means so marked, and at the ordinary temperatures and humidities of low- 

 level stations in our islands the two tables give more concordant results. 



Such divergencies might be explained were some influence neglected by one or both 

 of these distinguished men, which modified the results. Regnault suspected that the 

 speed of the wind would affect the temperature to which the wet bulb is lowered by 

 evaporation, and made observations to test this, but did not consider it necessary to 

 make any correction in ordinary circumstances. Sworykin t gives the influence of 

 wind on the factor necessary to convert wet bulb departures into dew-point departures 

 as follows : — - 



.Spued of wind in metres per second, . 0"85 



Percentage variation of factor, . . 71 '5 | 24 - 9 



1 



22-4 



2 

 13-3 



4 

 7 3 



6 

 5-0 



10 

 31 



00 



So that in calm, or almost calm weather, the condition of the air round the wet bulb is 

 markedly different from that of the surrounding air, which forms the atmosphere around 

 the bulb when there is current enough to constantly remove the water evaporated from 

 the muslin-covered surface of the bulb, j 



The divergencies in the tables arise from the observations on which they were based 

 not being comparable, those made in a calm being treated exactly as those made in 

 windy weather. This, too, explains why, at low temperatures and humidities, 



* Proceedings R.8.E., vol. xiii. p. 958. 



t Treated in Professor Assmann's paper, Ablmndluwj Kimiyl. Preuss. Meteorolotj. Inst., i. 234, 1892. 

 \ Reference may be made to the work of Professor IIazen and others in America, and to that of 

 Professor Assmann and others in Germany. 



