166 G. Siiteal: Psj/cliroinetric Formuld.. 



were obtained by kikinfi- the arithuietic means). Willi strong winds, 

 say from 15 to 20 miles an hour (about 25 ft. a second) upwards, the 

 result of 25 observations is that A - 0.00()69:K). Taking together all 

 those observations in which the air was distinctly in motion, the value 

 is 0.0007213. Thus it would seem that the value 0.00072 is satis- 

 factory for all conditions oi ■ ventilation, supposed occurring arbi- 

 trarily, while absolute calm should be avoided if possible. 



Tlie suggestion previously made that a ditference might be found 

 according to the direction of the wind is not contirmed by these obser- 

 vations. In 47 cases the wind was from the north (including N.E. 

 and N.W.), and for these A = O.0007186 ; in 42 cases the wind was 

 from the south, and the value comes 0.0007199. 



Any relations which may exist betAveen the value of A and the 

 temperature or humidity were completely masked by the wind-ett'ect, 

 and the number of observations was not sutticient to allow of a 

 separation. 



Effect of Soiled Coverings. 



That the nature of the evaporating surface does not att'ect the tem- 

 perature attained by the wet-bulb thermometer is a conclusion which 

 does not extend to the state of cleanliness of that surface. Tlie 

 necessity for frequent renewal or cleansing of the materials is well 

 recognised, and the observations which, as already mentioned, were 

 unintentionally made with soiled coverings, show that this regulation 

 is by no means unimportant. The different wet-bulbs agreed together 

 as closely as when clean, all being equally soiled, or at any rate 

 exposed to the same conditions, but the observations were discordant 

 among themselves, i.e., in the individual values of A, several were 

 erroneous to an extreme, and the wet-bulbs read in practically all* 

 cases too high. The value of A derived from the 'M ol)servatiojis was 

 O.O008684, with a probable error of 0.0000232, while with the two- 

 constant frirmula the result was rj = 0.9254 ± 0.0155, A - 0.0005892 

 ±0.0000631. The hindrance to evaporation wjis evidently large, 

 and the instrument in such a state is useless. 



In closing, 1 should like to offer my thanks ti> l)ofli Dr. Love ann 

 Professor Lyle for the kindly advice and assistaiuf whiih have always 

 been placed at my disposal. 



