19^ CELESTIAL MEASURINGS AND WEIGHINGS. 



called by astronomers)* can be ascertained. The first 

 consists in determining the exact situation which its 

 direction in space holds at all times of the year in rela- 

 tion to some plane, and to some line in that plane which 

 we have reason to consider as fixed j or at all events of 

 whose movements (exceedingly small in amount) we can 

 render an exact account. Such a plane is that in which 

 the earth revolves round the sun, or the ecliptic, and such 

 a line that of the equinoxes, and the astronomical process 

 employed is that by which the two elements technically 

 called the longitude and latitude of the star are determined. 

 This is in effect the process by which all celestial charts 

 are constructed and catalogues of stars made. Only for 

 this purpose the observations require to be made with 

 the very best instruments ; with the minutest attention to 

 everything which can affect their precision ; and with the 

 most rigorous application of an innumerable host of "cor- 

 rections" some large, some small, but of which the 

 smallest, neglected or erroneously applied, would be 

 quite sufficient to overlay and conceal from view the 

 minute quantity we are in search of. To give some idea 

 of the delicacies which have to be attended to in this in- 

 quiry, it will suffice to mention that the stability not only 

 of the instruments used and the masonry which supports 

 them, but of the very rock itself on which it is founded, 

 is found to be subject to annual fluctuations capable of 

 seriously affecting the result. So that it is only when 

 after a series of observations continued for several years 



What is technically called parallax^ is only the half of the total 

 annual apparent displacement 



