304 Bibliography. 



'52, upon Venus and Mars, with certain stars along their paths. The 

 observations upon Venus which will most command the attention of 

 the Expedition, will be differential measurements upon that planet, in 

 the morning and evening, while it is near the inferior conjunctions of 

 1850 and 1852. In like manner, Mars will be compared with its 

 neighboring stars near the times of opposition of that planet in 1849 

 and 1852. The object of these observations upon this planet, is a 

 more accurate determination of its parallax. 



To facilitate the observations and to secure concert of action, so that 

 the cooperators, in whatever part of the world, may, in observing the 

 planets, always use the same stars of comparison, Lieut. Gilliss has 

 prepared the accompanying charts and tables. Charts No. 1 to 5, in- 

 clusive, refer to Venus; 6 and 7 to Mars. They show the approxi- 

 mate places of the planets from day to day relatively to the stars, down 

 to the tenth magnitude, near their path. 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 the 

 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 stars marked W. C. are from the unpublished observations of the 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 Catalogue ; as they have not undergone their final reduc- t 



tions, 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 Ephemerides is to give the place of the star with accu- 

 racy sufficient merely to leave no doubt as to the identity of the par- 

 ticular star, which all observers are requested to use during the obser- 

 vations for the day thereby provided for. It is requested that those 

 who may have the goodnes to cooperate in these observations will ob- 

 serve the planets also, both for K. A. and Dec, at their meridian 

 passage. 



The order of observations, proposed by Lieut. Gilliss, is this: — Du- 

 ring the term of the Ephemeris of Mars, differential measurements 

 upon that planet, and the star of comparison for the day, will be com- 

 menced at two hours after the passage of the planet across the merid- 

 ian of Greenwich, and be continued for one hour and a half after the 

 star and planet shall have passed the meridian of Washington ; observ- 

 ing and comparing with the star, the North and South Limb of the 

 planet alternately. 



Both the planet and star of comparison will also be observed, with 

 the Meridian Circle, at their transit across the meridian of the Observa- 

 tory in Chile. The same course is proposed to be pursued, at meridian 

 transit, with regard to Venus and her star 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 



