xxviii ORIGIN AND OPERATIONS 



In like manner, Mars will be compared with its neighboring stars near the times of opposi- 

 tion of that planet in 1849 and 1852. The object of these observations upon this planet 

 more accurate determination of its parallax. 



To facilitate the observations, and to secure concert of action, so that the co-operators, in 

 whatever part of the world, may, in observing the planets, always use the same stars of com- 

 parison, Lieut. Gilliss has prepared the accompanying charts and tables : 



Charts Nos. 1 to 5, inclusive, refer to Venus ; 6 and 7 to Mars. They show the approximate 

 places of the planets from day to day relatively to the stars down to the tenth magnitude near 



their paths. 



In some parts of the paths of the planets, along which published catalogues do not afford 

 proper stars of comparison, special observations have been made with the large refractor of the 

 National Observatory ; the stars whose approximate places have been thus obtained are mapped 

 down along the planet's path. 



Tables 1 and 2 contain the ephemeris of the planets and stars of comparison. They give 

 the star of comparison for each day, and quote its magnitude, with its approximate mean 



place only. 



The stars marked W. C. are from the unpublished observations of the Washington cata- 

 logue ; as they have not undergone their final reductions, their declinations are only given to 

 the nearest 10". The other stars are designated by the initials or name of the catalogue from 

 which they are taken. 



In the ephemerides of the two planets and their neighboring stars, the mean places of the 

 stars for 1st January of the year for which the ephemerides are calculated, are given. The 

 object of such ephemeris is to give the place of the star with accuracy sufficient merely to leave 

 no doubt as to the identity of the particular star which all observers are requested to use during 

 the day thereby provided for. 



It is requested that those who may have the goodness to co-operate in these observations will 

 observe the planets also, both for right ascension and declination at their meridian passage. 



The order of observations proposed by Lieut. Gilliss is this : During the term of the epheme- 

 ris of Mars, differential measurements upon that planet and the star of comparison for the day 

 will be commenced at two hours after the passage of the planet across the meridian of Green- 

 wich, and be continued for one hour and a half after the star and planet shall have passed the 

 meridian of Washington, observing and comparing with the star the north and south limbs of 

 the planet alternately. 



Both the planet and its star of comparison will also be observed, with the meridian circle, at 

 their transits across the meridian of the observatory in Chile. 



. The same course is proposed to be pursued at meridian transit with regard to Venus and her 

 stars of comparison. 



Lieut. Gilliss proposes to commence the differential observations upon Venus and her star of 

 comparison as given in the ephemeris as early in the evening and morning, and to continue 

 them as long, as the light of the sun and the conditions of the atmosphere may admit. Owing 

 to the absence of stars of sufficient magnitude within 15 of the sun, an omission is made in the 

 ephemeris during the time that the planet will be within that distance of the sun. It is pro- 

 posed during such intervals to rely exclusively on meridian observations, both at the observa- 

 tory in Chile and elsewhere. 



The precise place at which the observatory is to be erected will not be decided upon until the 

 arrival there of the Expedition. 



Those astronomers who are disposed to forward the objects of the Expedition so far as to 

 co-operate with it in conducting an auxiliary series of observations, will perceive that the results 

 of their labors will be enhanced by using, whenever practicable, the stars of comparison which 

 Lieut. Gilliss has selected, and which are given in tables 1 and 2, and by following generally 

 the plan of observations proposed by him, and herein explained. 



