138 MR. W. N. SHAW ON HYGROMET1UC METHODS. 



paragraphs, which I have translated directly from his memoir : " A further influence, 

 of which no account was taken in the formula, lies in the muslin covering of the wet 

 thermometer. In establishing the formula no regard was had to any such effect. 

 KAMTZ has, however, called attention to the frequent case, which he had occasion to 

 observe, that the moist thermometer stood sometimes as high, sometimes higher, than 

 the dry, when a comparison with the hygrometer did not show a saturation of the air 

 with water vapour. He had investigated the cause of this phenomenon, and had 

 found it in the muslin covering of the wet thermometer. According to his experi- 

 ments at low temperatures, the wet thermometer stood 0'46 C. too high. 



" That this should be an effect of radiation, as is no doubt the case with dry 

 muslin, would require as a consequence that the radiation effect should increase with 

 increase of the difference (t t'). The fact is, however, that this effect in propor- 

 tionate comparisons diminishes (mehr verschwindei) as t t' becomes greater, at any 

 rate in moving air. This behaviour will require further searching investigation. I 

 will, however, at present mention that I am of opinion that this phenomenon is to be 

 ascribed to an ' inertia ' of the wet thermometer when near the saturation point of 

 air, so that in very moist air the evaporation does not take place rapidly enough in 

 relation to the in-rushing air. This is confirmed also by experiment. If account is 

 to be taken of this ' inertia' of the psychrometer in the formula, a correction can be 

 so applied to the psychrometer difference (t t'), that it is a maximum when 

 t t' = 0, and is chosen inversely proportional to the difference (t t'). If v be the 

 maximum value, this correction would be = v/(t t' + 1) ', if we, with KAMTZ, take 

 v = 0'5 C., this correction will only be of appreciable value up to differences of 9, 

 practically only up to those of 6." 



I have translated this directly, as the point is an important one, and this is the first 

 explanation of it that I have seen attempted, but I cannot say that I fully understand 

 the method of introducing PERNTEK'S correction. The quantity v is said to be the 

 maximum value of the correction, which I should take to be the temperature excess 

 of the wet bulb over the dry when the air is actually saturated, but this would 

 evidently not be the case when t t' = 0. The complete formula, as thus corrected, 

 stands thus 



= -Px 0-000630 



There is evidently something wrong with this, as the condition for saturation, viz , 

 Po Pn leads to the imaginary difference 



t-t'=--5{l v/-l}. 



No comparisons of dew-points computed by the formula with actual observations 

 are given, although the observations of BLANFORD, CHISTONI, and others are discussed 

 in the course of the paper, and the values of the constants deduced from them are given. 



