30 PROCEEDINGS OP THE AMEEICAN ACADEMY 



atmosphere. No difference in the spectrum was detected either by 

 myself or by Mr, W. H. Pickering after careful examination. 



This negative result should not be regarded as throwing doubt on 

 the positive results attained by so skilful an observer as Professor 

 Young, who is said to have detected the presence of aqueous vapor in 

 Venus. I have not yet seen the details of his observation, but his facili- 

 ties for making this observation were much greater than mine, and he 

 probably used a much higher dispersion. I satisfied myself that there 

 were no very marked absorption bands, and doubtless the phenomenon 

 is one which requires more careful preparation than we were permitted 

 to make without interfering with the other portions of our programme 

 to which, in preparing our plans, we had attached more importance. 



Diameter of Venus. 



The measurements of the diameter of Venus, mentioned in the first 

 paragraph of this communication, were made by Professor William A. 

 Rogers and by Mr. S. C. Chandler, Jr. The subjoined reports from 

 these gentlemen furnish the account of the work undertaken. In these 

 reports, Mr. Chandler's telescope, mounted in the West Dome, has 

 been called the West Equatorial. 



Report hy William A. Rogers. 



The following method for the determination of the diameter of a 

 planet was first employed by the writer in 1877, having been used in 

 the determination of the diameter of Mars. 



Let: — 



Xq = a line ruled upon glass and set in the direction of diurnal motion. 



a-j ^ a line ruled at a given angle, i, with respect toXy, and reckoned 



from east to west. 

 3-2 = a line ruled at the angk (180° — i) with respect to x^. 

 y =^ a line ruled at right angles to x^ and bisecting the angle formed 



between x^ and x^. 

 Tj = the observed time of transit of the preceding limb of the 



planet over x^. 

 Tj = the time of transit of the following limb over x^. 

 Tg, T^ = the corresponding times over x.^. 

 D =: the diameter of the planet. 



Then : 



Z> = 15 cos 8 (tjj — Tj) sin i = 15 cos 8 (t^ — r^) sin i 



