METEOROLOGY. 517 



trials.* Since now the factor A consists of the constant invariable 

 part ^— — , and the variable part 



\jkp6 



formula thus (see the formula H) 



part TS --— , and the variable part S-— we can, since v - = 1 write the 

 DApa DApa D 



psr 



p "= ih -j* L 1+ 



lir 

 aDSp 



(t-f) (I) 



where a denotes a new constant that under the assumption of a uniform 

 ventilation and constant barometric pressure should be invariable. 



From this we see that the term depending on radiation can indeed 

 be diminished by ventilation, but certainly not so much that it can 

 he wholly neglected. 



It will not be superfluous at this place to indicate how very erroneous 

 it has been to neglect the influence of radiation in the deduction of the 

 psychrometer formula. In fact it proves to be for still air quite as 

 large as the theoretical factor in August's formula, if one assumes a 

 spherical thermometer bulb of 0.57 centimeters radius, as is the case at 

 the observing stations of the Vienna K. K. Central Institute for Meteor- 

 ology and Terrestrial Magnetism. 



If we would possess a uniform formula for all the various ther- 

 mometers, we mast seek a method of carrying out the suggestion of 

 Kiimtz.t He remarks, "As a pendulum occupies only approximately 

 equal times for a vibration over large and small arcs, equally so differ- 

 ent thermometers give only approximately equal results. As with the 

 pendulum the arcs must all be reduced to one of infinitely small ampli- 

 tude, so with the thermometer we must undertake a reduction to an 

 infinitely small one." 



We can, however, iu using the ventilation-psych rometer, consider 

 this reduction as already partly made, since in that instrument the term 

 that contains the influence of radiation is materially diminished. 



A further influence that has not at all been considered in the formula, 

 is that of the muslin cover of the wet-bulb thermometer. In the de- 

 duction of the formula no consideration was given to this. Now, Kamtz 

 (being made observant by reason of the frequent cases that he had 

 occasion to observe J where the wet thermometer stood sometimes as 

 high, and sometimes higher than the dry, when a comparison with the 

 hair hygrometer showed the air to be not saturated with vapor) has in- 

 vestigated the cause of the phenomenon, and found it in the muslin 

 covering of the wet thermometer. According to his experiments, at low 

 temperature the wet thermometer stood too high by 0.40° C. 



* This is the value that Regnault gave as long ago as 1845. The form of the psy- 

 chrometer formula, as it has since then beeu almost universally adopted, contains 

 this value ; it reads 



Po = Pl - 0.480 6 4=^,P 



tKantz Rcpertorium fur Meleorologie, bd. n, p. 56. 

 t Kantz llepertorium, bd. n, p. 54. 



