August 5, 1910] 



SCIENCE 



175 



At the University of Illinois assistant pro- 

 fessors have been appointed as follows : Dr. 

 John Byrnie Shaw, mathematics; Dr. George 

 F. Arps, of Indiana University, psychology; 

 Mr. David Varon, of New York City, architec- 

 tural design, and William Thomas Bawden, of 

 the State Normal School, Normal, 111., engi- 

 neering. 



Recent appointments in the School of 

 Mines of the University of Pittsburgh are as 

 follows: Horatio 0. Ray, B.S., instructor in 

 metallurgy; Harry N. Eaton, A.M., instructor 

 in geology and petrography ; Henry Leighton, 

 A.B., instructor in mining geology and min- 

 eralogy; Harry B. Meller, E.M., instructor in 

 mining. 



The Toronto correspondent of the New 

 York Evening Post states that appointments 

 at the university have been made as follows: 

 Dr. J. A. Arnyot, director of the laboratory of 

 the provincial board of health, to be professor 

 of hygiene, in succession to Dr. William 0. 

 Wright, resigned; H. E. T. Haultain, pro- 

 fessor of the new chair of mining engineering ; 

 Dr. W. H. Piersol, associate professor of his- 

 tology and embryology; Dr. K. C. Mcllwraith, 

 associate professor of obstetrics ; S. E. Creaser, 

 lecturer in surveying; W. W. Frey and J. J. 

 Traill, lecturers in mechanical engineering; 

 J. H. White, lecturer in forestry and botany; 

 Alex. McLean, lecturer in geology. 



Dr. Howard L. Bronson, assistant professor 

 of physics in McGill University, has been ap- 

 pointed to the chair of physics in Dalhousie 

 University, Halifax, vacated by the resigna- 

 tion of Professor A. S. McKenzie to accept a 

 chair in the Stevens Institute of Technology. 



DISCUSSION AND CORRESPONDENCE 



'the bearing of psychrometer readings on 

 measurements of martian aqueous 



VAPOR 



To THE Editor of Science: Referring to 

 Dr. Abbot's letter to Director Percival Lowell,' 

 the point at issue can not be settled by psy- 

 chrometer readings, taken merely at the 

 earth's surface. 



Dr. Slipher, in commenting upon the Flag- 

 staff Mars-moon spectrogram Rm 3050, taken 



'Science, June 24, 1910, p. 987. 



at the Lowell Observatory, January 15, 1908, 

 when the psychrometer indicated 1.30 grains 

 of water-vapor per cubic foot of air, and 

 comparing it with plate Rm 3062, taken on 

 January 21, with a vapor-reading of 1.02 

 grains, says: 



A long series of exposures to the spectrum of 

 the moon at different altitudes, made on the same 

 night [January 15] . . . verify the lunar images 

 of the Mars spectrogram in showing that the 

 moisture in our air was relatively very much less 

 than for plate Rm 3062, notwithstanding the 

 meteorological records to the contrary. The 

 strength of the a band depends upon the actual 

 amount of aqueous vapor in the light path and is, 

 therefore, a very reliable measure, whereas the 

 meteorological observations can not be reliable for 

 they depend upon the moisture in a small sample 

 of air at the earth's surface which may be very 

 different from what it is a short distance above.^ 



Director Campbell says: 



It would be interesting to know how much 

 vapor was traversed by the rays of Mars and the 

 moon when the spectra were recording themselves 

 on the sensitive plates, but to speculate on the 

 subject, from the thermometer readings, seems 

 useless, in view of the unknown law of distribu- 

 tion of vapor in the strata above the thermom- 

 eters. The vapor bands in the spectrograms them- 

 selves furnish the only known rational method 

 of estimating the quantity of vapor traversed.^ 



The same principle is recognized in my 

 " Reply to Campbell's criticism," where I 

 say: 



It is the distribution of moisture through the 

 entire air column that we should like to know, 

 and this is hardly affected by such surface changes 

 as occur in an arid region. . . . Any great accu- 

 racy in the determination of surface humidity 

 would be labor wasted for the present purpose, 

 A mean diurnal, or possibly a mean monthly 

 humidity may be quite accurate enough.* 



^V. M. Slipher, "The Spectrum of Mars," As 

 trophysical Journal, Vol. 27, No. 5, p. 401, De 

 cember, 1908. 



^ W. W. Campbell, " The Spectrum of Mars as 

 Observed by the Crocker Expedition to Mt. Whit 

 ney," Lick Observatory Bulletin, No. 169, p. 153 

 October 1, 1909. 



* Frank W. Very, " Water Vapor on Mars — ^Re 

 ply to Campbell's Criticism," Lowell Observatory 

 Bulletin, No. 43, p. 240. 



