134 



KNOWLEDGE. 



April. 1911. 



view, using a trough of mercun- for the reflected 

 image. The use of this method is of great import- 

 ance in the drscussion of fundamental observations. 

 For this class of observations calm nights are 

 requisite, or those with extremelv little wind. 



Old and New Work. 



These remarks aim at explaining the reason of the 

 plan of the fundamental work, the studv of all the 

 instrumental and personal errors, the determination 

 of the position of the instrument without using an 

 adopted position of any star, and the determination 

 of the correction and the change or rate of the clock's 

 error by groups of stars in opposite parts of the skv. 



This method entails more work, in order to 

 measure from one star to another, than the method 

 mostly in use. It is necessarv to continue the 

 observations at least for thirt\-six hours, in order to 

 include three groups of stars. The calculations are 

 very heavv. 



The corrections oi the position of the instrument 

 in a long period ma\- alter sensibh", and it is 

 necessary to study well the progress of all errors. 

 The arcs between the stars are large and the errors 

 of the observations are relativeh' increased. Finallv, 

 it is necessary to observe in the da\- time when the 

 brighter stars only are within the limits of vision. 



For these reasons, and because one can observe 

 the small or fainter stars onlv at night, the method 

 mostl\- used is to fix. by differential means, the 

 positions of these fainter objects with reference to 

 the principal stars in their vicinitx". 



Inthis manner we have had the principal catalogues. 

 which have been made by degrees, from one part of 

 the sky to the other, just as one has fixed the points 

 on the earth by the difference of longitude between 

 each point and another point about the first or 

 primary. All the longitudes have a single base at 

 last : though all the differences have not been directlv 

 measured from the base, it is adopted as the origin 

 of the system of longitudes. 



The errors of the differential method can be small 

 in exact observations, yet they have some systematic 

 corrections which increase from one epoch to another. 



Now and then it is necessar\' to revise the scheme, 

 compare the fundamental data, and rectify all the 

 area, with results of great precision and with more 

 complete calculations. In this way the results of the 

 old observations are improved, and these then enter 

 into the modern calculations with less svstematic 

 error. Astronomers whom we succeed used those 

 results in the calculations of the movements of the 

 stars and in the movement of the solar system. 



Fundamental Method Extended. 



The stars which we observe bv the tundamental 

 method are now reckoned among those that are better 

 known or determined in the sky. In the first class 

 are included the stars used in this method for clock 

 corrections. The positions of those in the first class. 

 when once well revised are used for fixing the positions 

 of the second class, which is of greater extent. 



containing the principal stars of all parts of the sky. 

 A third class, in our plan, includes a great number of 

 stars, until now of less value in fundamental work, 

 all well fixed or connected b\ simultaneous observa- 

 tions. In these classes we shall have altogether 

 1600 fundamental stars. 



By means of the positions of the fundamental stars, 

 one calculates the positions of all the other stars which 

 one observes on this plan. We include the greater 

 part of the stars which have been observed before the 

 epoch 1S75'0. The positions of these stars serve 

 to compute man\' proper motions, and for fixing the 

 direction and quantitv of the motion of the solar 

 system. We estimate that we shall obser\e on our 

 plan 15.000 stars in all. 



They are of all degrees of brightness from the first 

 magnitude, besides those more difficult to observe 

 w ith a telescope of this power. Observations of all 

 are to be found in the catalogues of the past forty 

 years and in the older ones. So, as the old catalogues 

 ser\-e as a basis for the present work, so also will 

 the observations on our jilan similarh' ser\e for 

 future calculations. 



C"LnL\TE AND WoRK. 



The development of this project has given good 

 results in the observations obtained since the 

 commencement of our scheme of work. In the first 

 complete \"ear we have made more than sixtv-two 

 thousand observations. This number has never been 

 reached, for this class of observations, by an\' other 

 observatorw .\s the greater part of the work has 

 alreadv been concluded, the task will be easier now. 



For the most part the climate of San Luiz has 

 been good. In the first vear we had three hundred 

 nights during which one could observe for some hours 

 at least. More than two hundred nights were clear 

 during all hours. Under the usual condition of the 

 sky we could generally reckon upon seventy per cent, 

 of the time for suitable astronomical observations. 



A series or c\"cle of fundamental observations is 

 one-hundred-and-twenty hours, as a general rule, but 

 the observations are continued without intermission. 

 One makes six observations successively of a group 

 of stars and six observations of another group at a 

 distance of twelve hours from the first. These groups 

 are usualh" made in the evening and early dawn. 

 The same observer continues during some hours 

 at night in order to include the stars which passed 

 in those hours. Other observers follow during the 

 last part of the night for fixing other stars with the 

 principal stars. They are occupied from twelve to 

 ten and six hours for the observations during the 

 twentv-four hours. 



The commission has consisted of ten persons for 

 four months only: during the greater part of the 

 work we had six or eight. W^e are occupied with 

 the computations as well, but these will require 

 manv \'ears before being accomplished, and the 

 results of the preliminarv calculations are sent to 

 North .America, where the\" will be concluded at the 

 Alban\- Observatorw 



