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On the Number of Comets in the Solar System ; on their 

 Light, whether Reflected or Emitted ; and on the Comet which 

 will pass its Perihelion in November 1835. By M. Akago. * 



1. On the Number of Comets. 



Although the question as to the number of comets has 

 received a large share of the attention of astronomers, yet 

 satisfactory observations are comparatively so recent, that we 

 cannot attempt, to offer more than the statement of certain pro- 

 babilities on the point. 



M. Lalande, in 1773, calculated that there were about three 

 hundred in our system. On the present occasion, I purpose 

 again to go over the ground by which he arrived at this conclu- 

 sion, always, however, applying his reasoning to the numerical 

 data which have been supplied us by the observations which 

 have been made from the year 1800 up to the year 1830. 



In this interval of thirty years, thirty-eight comets have 

 been observed, without taking into account the appearances of 

 the comet of 1200 days, nor that of six years and three quarters. 

 This gives a supply at the rate of four comets for every three 

 years. 



If the duration of the comets which we now see was not more 

 than 200 years, we might learn from the historians and chrono- 

 logists the traces of the previous appearance of every one of 

 them ; for since the year 1600, all the celestial phenomena have 

 been noted with sufficient accuracy. And we may here take the 

 liberty of adding, that, whenever it was possible to observe any 

 of these luminaries for a period of some weeks, the ellipticity 

 of their orbits would be sensible, provided the duration of their 

 revolution did not exceed 300 years. 



Let us adopt, then, the period of 300 as the mean term for 

 the time occupied by a comet in returning to its perihelion. 

 Starting from any given epoch, so long as this period of 300 

 years has not expired, we should constantly see new comets ap- 

 pear ; and this period once elapsed, the same luminaries would 

 again return, though it might be in a different order. 



The comets being thus all new ones during the period of 300 



* The above observations on Comets are taken from the third edition of 

 Arago " Des Cometes en Generate," and the last volume of the Annuaire. 



