7S Mechartical Philosophy, [Chap. I, 



and many curious facts respecting them discovered, 

 and received ample illustration. The learned and 

 indcflitiffable labours of father Boscovich, and of 

 M. de la Place, for determining the orbits of 

 comets, have been long known and praised by 

 astronomers. The great works of M. Pingre, and 

 of sir Henry Engiefield, in this branch of astro- 

 nomical philosophy, are entitled to a place among 

 the most full and useful of those which have ap- 

 peared on the subject*. But beside what has 

 been effected by the useful inquiries of these gen- 

 tlemen, the observations of many others, and par- 

 ticularly of de Lalande, and his countrymen. 

 Messier and Mechain, and also of Burckhardt, an 

 illustrious German astronomer, have contributed 

 much to extend our knowledge of comets. It is 

 further worthy of remark, that the difficulty of 

 making observations on comets has been, w^ithin 

 a iQw years, greatly diminished. The methods of 

 calculating their elements are now short and easy, 

 in comparison with what they were half a century 

 ago. Operations which then occupied many days, 

 may now be dispatched with accuracy, in a few 

 hours. 



The importance of accurate observations on the 

 Jixed stars, in order to ascertain their motion, 

 places, and relative circumstances, is known to 



V. Ill probably soon receive a solution, if the spirit of zeal and in- 

 dustry should continue \vhich at present animates many European 

 philosophers. 



* Treatise on Comets, by M. Pingre, 2 vols, 4to, Paris, 1783. 

 The Detenu inntion of the Orbits of Comets, accordivg to the Methods, 

 of father i)o^Qo\n:\\y and M. de la Place, "with new and complete 

 Tables, by sir Henry Englefield, 4to, 1799. 



