METEOROLOGY AND ALLIED SUBJECTS. 259 



used at elevated stations; the co-eflBcieut of a? varied from —1.199 at 

 ISO'""' to —0,920 at 400'"'". (Z. 0. G. M., XVI, 1881, p. 273.) 



Pernter has given excellent rales as to the use of the aneroid, and 

 urges that observers be not misled by these elaborate inquiries into con- 

 sidering the aneroid as an independent instrument, but that it must re- 

 tain its place as a means of interpolation only and that the mercurial 

 must be used daily as a check ; the instrument is always received after 

 the degrees of low or high pressure have been marked upon it. {Z. 0. 

 G. ]\L, XVI, 1881, p. 273.) 



Hottinger & Co. have also constructed a self-recording thermometer 

 and hygrometer that, as well as their barometer, commend themselves 

 on account of their simplicity, accuracy, and convenience. There are 

 three examj^les of the apparatus being investigated at the Zurich Ob- 

 servatory ; through a small range of temperature and moisture it gave 

 the following results : 



One degree of tem]ierature is represented in the records by 2.38"™, 

 3.2""", l.C'""\ respectively. 



The mean discordance of the corrected records was rt 0.12°, i 0.14°, 

 ± 0.18O C, respectively. 



For the hygrograph the mean errors for the percentages of relative 

 humidity were ± 1.2, i 1.7, ± l.C, respectively. {Z. 0. G. il/., XVI, 1881 

 p. 283.) 



Colding has endeavored to ascertain the true velocity of the wind by 

 careful observation of the i^aths of definite masses of smoke issuing 

 from tall chimneys. For Copenhagen during 1878 and 1879, he finds 

 the average velocity 5.5™ per second ; the anemometer records at neigh- 

 boring coast stations of the Deutsche Seewarte give 5.1™ and 5.5"". 



Observers in favorable localities would do well to supplement their 

 own records by this class of observations, it being well known that the 

 wind force and direction at a few hundred feet altitude is very different 

 from that at the ordinarj- level of observers and anemometers. {Z. 0. G. 

 il/., XVI, 1881, p. 270.)^ 



Maxwell has published in the article "Diffusion" in the ninth edition 

 of the "Encyclopjedia Britannica" an original demonstration of a for- 

 mula for the wet-bulb thermometer based on the assumption that the 

 surrounding air is quiet and that the dissipation of heat and vapor 

 takes place wholly by processes of radiation, conduction and dift'usion ; 

 whereas, August and Eeguault based their forraulai on the assumption 

 that the principal agency is convection due to air-currents. Maxwell 

 is led to the following formula: 



^« =P^ - L^ \ D+4;rC^SD 1 ^^^ " ^'^ 



where p^ is the desired tension of vapor in an atmosphere whose tem- 

 perature is So, and j^^i is the tension corresponding to the temperature di 

 of the wet-bulb. P is the prevailing barometric pressure, and the other 



