■of the Transit Instrument. 429 



by another, placed towards the North ; now although this seems 

 plausible enough in Theory, yet when put into practice, it will 

 be found to bring with it, it's Inconveniences. Thus, the Meri- 

 dian wire will occasionally bisect the one^ but not the other'; 

 and as each Mark has been erected under similar Precautions, 

 it becomes a matter of no small difficulty to decide^ which is 

 rightj or which is wrong ; nor can the Instrument with any pro- 

 priety be called in to settle the Dispute, seeing that the very 

 act of placing it under such guidance, impliesy that it is con- 

 sidered incapable of taking care of itself. Again, the Instru- 

 ment will now and then deviate from bothy and when corrected 

 for one, will be found to deviate from the other ; hence arises 

 another Difficulty, from which, as in the former instance, Side- 

 real Observations alone can extricate us. 



In alluding, however, to the fixed Stars, those onli/, whose 

 right ascensions are well settled, are fitted for the purpose, 

 and with such, the Greenwich Catalogue, presents us. 



But they, in consequence of various causes, are constantly 

 varying in apparent Right Ascension ; Corrections, therefore, the 

 result of these, must be applied to the Star's mean R. A. be- 

 fore its apparent right ascension can be determined. Now 

 these are Calculations requiring the Sacrifice of much Time and 

 Labour ; to obviate which, the late Astronomer Royal published 

 his seventeenth and eighteenth tables. The first of these, 

 contains the Sum of the corrections, in Aberration, Precession, 

 and Solar Inequality of Precession ; the second, that, arising 

 from Lunar Nutation : the former is calculated, for every tenth 

 day of the Year, the latter for every tenth degree of the Moon's 

 Node ; and when the equations are required, for any other day 

 of the year, or any other place of the Node, they must be found 

 by proportion. 



Now, although the labour is thus materially diminished, 

 still, what remains is far from inconsiderable, and it is to 

 this circumstance, that we must attribute the observations 

 of these Stars being by no means so general as might be 

 "wished. 



