PREFACE. 



AFTER the laws of planetary motion were discovered, the genius of KEPLER 

 was not without resources for deriving from observations the elements of mo 

 tion of individual planets. TYCHO BRAKE, by whom practical astronomy had 

 been carried to a degree of perfection before unknown, had observed all the 

 planets through a long series of years with the greatest care, and with so 

 much perseverance, that there remained to KEPLER, the most worthy inheritor 

 of such a repository, the trouble only of selecting what might seem suited 

 to any special purpose. The mean motions of the planets already deter 

 mined with great precision by means of very ancient observations diminished 

 riot a little this labor. 



Astronomers who, subsequently to KEPLER, endeavored to determine still 

 more accurately the orbits of the planets with the aid of more recent or 

 better observations, enjoyed the same or even greater facilities. For the 

 problem was no longer to deduce elements wholly unknown, but only 

 slightly to correct those already known, and to define them within narrower 

 limits. 



The principle of universal gravitation discovered by the illustrious NEWTON 



b (ix) 



