AIDS OF THE NEWTONIAN PERIOD. 291 



of the heavens as known by previous researches, 

 implies a great amount of calculation. The exact 

 places of the stars at some standard period are 

 recorded in Catalogues; their movements, accord- 

 ing to the laws hitherto detected, are arranged in 

 Tables ; and if these tables are applied to predict 

 the numbers which observation on each day ought 

 to give, they form Ephemerides. Thus the cata- 

 logues of fixed stars of Flamsteed, of Piazzi, of 

 Maskelyne, of the Astronomical Society, are the 

 basis of all observation. To these are applied the 

 Corrections for Refraction of Bradley or Bessel, 

 and those for Aberration, for Nutation, for Preces- 

 sion, of the best modern astronomers. The obser- 

 vations so corrected enable the observer to satisfy 

 himself of the delicacy and fidelity of his measures 

 of time and space ; his Clocks and his Arcs. But 

 this being done, different stars so observed can be 

 compared with each other, and the astronomer can 

 then endeaveur further to correct his fundamental 

 Elements ; his Catalogue, or his Tables of Correc- 

 tions. In these Tables, though previous discovery 

 has ascertained the law, yet the exact quantity, the 

 constant or coefficient of the formula, can be exactly 

 fixed only by numerous observations and compa- 

 risons. This is a labour which is still going on, 

 and in which there are differences of opinion on 

 almost every point; but the amount of these dif- 

 ferences is the strongest evidence of the certainty 

 and exactness of those doctrines in which all agree. 



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