142 CARNEGIK INSTITUTION 



most latitude possible is desirable. Perhaps the southern part of the 

 Argentine Republic offers what is, on the whole, the most practicable 

 compromise (also the possibility of existence for several years). 



As closely related to the fundamental determinations and as a 

 work to be accomplished with the same instrument it is very desir- 

 able, as you propose, that there should be complete observation of 

 all stars south of — 20° and to the seventh magnitude, inclusive, 

 four times for each star (equally distributed in the two positions of 

 the instrument, and preferably according to the example set by the 

 Pulkova series for 1855, with exchange of objective and ocular). 

 In accomplishing this the period of observation will be scarcely 

 lengthened and a second work of the first importance would be pro- 

 duced. 



Of still greater importance than this second work, attached to 

 No. I, is the continuation of the astrographic zones, the proper and 

 speedy observation of all stars down to the ninth magnitude upon 

 the southern sky (No. 4). In Cordoba this work would have been 

 extended beyond — 32°, but since Dr. Thome has now undertaken 

 a part in the photographic chart it is very desirable that others 

 should undertake the continuation of the zones. For this purpose, 

 in addition to the meridian circle for No. i, a second instrument 

 would naturally be required; but I do not see the use of giving to 

 this instrument a construction of the form you have in mii^d. The 

 ordinary meridian circle is entirelj' suited for zone observations, and 

 in any half way favorable climate with such an instrument two ob- 

 servers (one at the ocular and the other at the microscopes) could 

 with ease make 10,000 observations annually at less than — 40° of 

 latitude where one has sufficiently long nights at all seasons; two 

 sets of observers could make 20,000 observations annuall3\ 



No. 2 is also a task of great importance. The determination of 

 the mean distance of the stars of various orders of magnitude is 

 necessary, in order to provide a firm foundation for investigations 

 into the structure of the stellar universe. But it appears to me im- 

 possible to reach this result otherwise than when one investigates 

 the parallax of each single star of the brighter orders of magnitude, 

 together with a sufficient number of the following orders, as far 

 down as the means of measurement will permit. Dr. De Ball has 

 planned such an undertaking and is corresponding in relation to it 

 with others who have heliometers at their disposal — among others 

 with Dr. Elkin. Therefore I will not go into further particulars, 

 and will only remark that it goes without saying that it would be 



