OBSEEVATOEY AT COEDOBA, ARGENTINE EEPUBLIC. 273 



have had to deal with them, and which are depicted alike iu no two 

 different maps or globes within my knowledge. I have now re-arranged 

 the whole system in such a manner that the boundaries of the constella- 

 tions shall be formed, so far as possible, by meridians and parallels. of 

 declination, and have found it practicable to arrange this with almost 

 insignificant disturbance of the nomenclature of the principal stais. 

 To this portion of the labor, also, I attach considerable value. 



The meridian circle possesses essentially the same optical power as 

 the instruments employed by Bessel and Argelauder, the object-glass 

 having an aperture of 4^ Paris inches. But methods of observation 

 have made considerable advance in twenty years, and this new iustrumeut 

 is supplied with varous conveniences which the others did not possess. 

 The principal difference of method, however, is in the employment of 

 the chronographic method of observing transits, the instants of these 

 being registered by telegraphic signals upon a cylinder revolving at 

 a uniform rate. The fundamental plan of all the zone observations of 

 which 1 have spoken consists in restricting the vertical motion of the 

 telescope to narrow limits proviously assigned, and then determining 

 the moment of transit and the declination of every star that traverses 

 the field within these limits, which of course regulate the width of a 

 strip or "zone" of the heavens, whose length is determined by the du- 

 ration of the process. It is manifest that the width of the zone can be so 

 chosen that only a small portion of the stars of sufficient magnitude 

 can escape detection. Thus, beginning each zone where the adjacent 

 one ends, the whole region iu question is gradually explored. 



The most essential point iu which the plan of my undertaking differed 

 from that of previous observers, is, that it was my aim to make the 

 determinations absolute, instead of relative. The principle adopted more 

 or less comx)letely in former series has been, in fact, to observe an entire 

 zone in such a way as to determine the differences of the several stars, 

 among themselves, and then, identifying those whose positions may be 

 found in catalogues already existing, to calculate the places of the new 

 stars from those of the others. Such had been my own original plan ; 

 but I soon became convinced that a sufficient number of star-places of 

 the needful precision was not accessible, and that it was desirable to 

 keep the work independent of any previous catalogue, aiming at what 

 is called an absolute determination of the stars observed — that is to say, 

 an entire independence of the work of all other astronomers, outside 

 of the data in the astronomical almanacs. This implied a great 

 increase of labor, since it would demand nearly an hour of addi- 

 tional observations, before and after every zone, for the sole purpose of 

 ascertaining the needful corrections to the indications of the instrument 

 and the clock, which vary appreciably from hour to hour. It likewise 

 entailed much additional labor in the comj)utations; and it became 

 necessary to jjrepare for our use in Cordoba the daily places of funda- 

 mental polar stars, which northern observers find calculated to their 

 18 s 



