Memoir on the approaching Return of Hall e^/^ Comet. 47 



Hon passage of Halley's comet next year, it becomes an object 

 of greater interest to observe it as soon as possible, and we 

 are naturally led to inquire whether it will not be visible 

 during the winter, or early in the spring of 1 835. Ferrer and 

 Wisniewsky had the good fortune to rediscover the comet of 

 1811 in July and August 1812, when it was at a greater di- 

 stance from the sun, and at least at as great a distance from 

 the earth, as Halley's will be in February and March 1835. 

 Whether it will be visible or not depends principally, as has 

 been often noticed, on its distance from the sun. A comet 

 does not escape observation in our telescopes from the small- 

 ness of its size, but from insufficient brightness to enable it to 

 be discerned in the sky. I am indeed far from supposing that 

 Halley's comet is as large, or under similar circumstances as 

 easily seen, as the splendid comet of 1811; but 1st, All the 

 former observers of Halley's comet represent the head as par- 

 ticularly brilliant. The nucleus resembled a fixed star [Pin- 

 gre, i. p. 460). Hevel says of its appearance in 1682, (Annus 

 Climactericus, p. 123.) "Toto apparitionis tempore lucidius et 

 aliquanto maj us caput exhibuit quam praecedensille anno 1681." 

 On the 22nd of September, he observes (p. 121.), "conspectum 

 tamen est caput cometae tubo optico ad exordium ipsum solis, 

 ob clarissimiim nucleum quem in meditullio referebat." Robert 

 Hooke also, [Posthumous fVorks, p. 161.) with others, was able 

 to see the comet on the 4th (14th) of September even till the 

 time of its setting. " I was able to see it almost to the very 

 horizon, even till it went behind a steeple, a little above the 

 tops of the houses, though the smoke much thickened the air." 

 2ud. About the middle of March 1835 it will be more strongly 

 illuminated by the sun, in the proportion of 8 : 5, than the comet 

 of 1811 was on the 17th of August, when Wisniewsky finally 

 saw and observed it. And 3rd, which is of most importance, 

 Ferrer found the latter comet on the 10th of July 1812 with a 

 comet-sweeper, and Wisniewsky was still able to observe it on 

 the 17th of August with a common 3 J feet achromatic; whereas 

 in searching for Halley's comet there is nothing to prevent us 

 from employing great refractors or reflectors which render 

 even the faintest nebulae visible. 



When I refer to the appearance of Halley's comet in 1682, 

 I assume, indeed, that it has not sustained since that time 

 any sensible diminution of mass or matter. Many astro- 

 nomers consider a gradual diminution of the matter of 

 comets to be probable, since such of them as appear with 

 tails must throw off and lose a great portion of the matter of 

 their tails at each return to their perihelia. Experience has 

 given no information on this point with respect to Halley's 

 comet Although in 1607, and at its last apparition in 1759, 



