SEcr.VIII.] Astronomy. 77 



some centre, which may be an opaque mass of 

 matter*. 



At the close of the seventeenth century, the re- 

 spective distances of the several planets from the 

 sun were far from being accurately determined. 

 These, by successive observations, have been since 

 ascertained, with a great degree of precision ; and 

 the various astronomical uses which this knowledge 

 is calculated to subserve, have been displayed in 

 the most satisfactory manner. Particularly the 

 observations made by many philosophers on the 

 transits of Venus and Mercury, which the eigh- 

 teenth century exhibited, have thrown much liglit 

 on this subject, and on several questions of great 

 importance in astronomy. 



It is but a few years since our knovvlcdgc of 

 Cornets Vvas in its infancy. Dr. Halley, at the be- 

 ginning of the period under consideration, made 

 the first attempt to give a systematic view of this 

 part of the science in his Synopsis Astronomic 

 Cometicce, published in 1705. But his inquiries 

 concerning these excentric bodies, though inge- 

 nious and highly valuable, were far from being 

 adequate or satisfactory. By the labours of mo- 

 dern astronomers, our acquaintance with the co- 

 mets has been wonderfully extended. Sixty-eight 

 new ones have been observed ; the return of many 

 of them has been ascertained and demonstrated f, 



* See Philosophical Transactions, vol. Ixxix. 



t M. de la Lande, in his Histoiy of jUtronoiij, for the year 

 1801, intimates, that the observations which took place in the 

 course of that year have made it questionable whether the doc- 

 trine long entertained, and con^iidered by him and other astrono- 

 mers as settled, that comets revohe, be not erroneous. This doubt 



