OBSERVATIONS. 



H. OBSERVATIONS OF MARS I. WITH THE ATHENS MERIDIAN-CIRCLE. 



The observations of Mars at Athens were made by Professor Bouris, then Director of the 

 observatory at that place, for the special purpose of proving useful in connection with the labors 

 of Lieut. Gilliss at Santiago, the meridian-circle being the only instrument at the observatory 

 available for the purpose. A full description of the measurements and their results, together 

 with an extended discussion, is given by Mr. Bouris in the Astronomische Nachrichten, vol. 

 XXXVII, pp'. 153-188. On only four nights was the observation of /9 Tauri omitted ; and, 

 in general, the choice of other stars was guided by the programme of Jfars-culminating stars 

 in the London Nautical Almanac for Is49. The readings of thealhidade-level and the correc- 

 tions for refraction computed in triplicate by Mr. Bouris are there given, and after such pains- 

 taking zeal on his part, it has seemed superfluous to renew any part of the computation. I 

 have consequently adopted the values of the correction for refraction as given by this astronomer. 



The only typographical errors to be noticed are : 



Page 165, NOT. 26 

 Dec. 14 



167, Jan. 2 

 " 3 

 " 6 



168, 6 



169, IS 



5 Gcminornin, Circle-reading, for 346O 30' 44".63, read 346 29'44".63. 

 1937 B.A C., Level, for 2".08, nod 2".80. 

 for 1648 Hist. C61., rwi<fM648 B.A.C. 



10669, H. C. Result, for 34".50, read 31".50. 



Mars N. Circle-reading, for 348 17' 21".50, read 348 18' 21".50. 



171, Feb. 1- 10, for 1754, Hist. Cel., read 1754 B. A. C. 



The diaphragm of the Athens circle was provided with two fixed horizontal threads, at a dis- 

 tance from one another which Mr. Bouris gives as 26". To insure variety of circumstance, and 

 for other reasons mentioned in his article already cited, Professor Bouris adopted the practice of 

 observing with the two threads alternately, using the one on the even, the other on the odd days 

 of the month, analogously to the rule which he followed for guiding his selection of the limb 

 to be observed. 



The first question for consideration, therefore, is to deduce from the materials furnished the 

 best value for the distance of these threads. The following table presents in a compact form the 

 data as obtained from the values of the zenith-distance of the zero-point, afforded by the indi- 

 vidual observations. Denoting the two threads by the letters A and B, the former being the 

 upper, (or that one which furnishes the larger circle-reading for the observation, and the smaller 

 for the zenith-distance ot the zero-point,) the table will explain itself. The columns headed 

 "Sum" present the sum-total of the seconds, belonging to the distance upon the graduated arc. 

 From the sums at the foot of the columns, we deduce the adopted value of the thread-interval 

 27". 299, which has been employed through the whole series ; careful scrutiny having failed to 

 detect either any gradual or any sudden and permanent change in this value during the entire 

 series of jtfors-observations. 



