494 PHYSICAL CONSTITUTION OP COMETS. 



Arago assigns their length at about ten millions of miles, others have estimated 

 it. at twenty-three millions of miles. The following description of it is taken 

 from the memoirs of the Academy of Sciences for ]744. It was first seen at 

 Lausanne, in Switzerland, December 13, 1743: from that, period it increased 

 in brightness and magnitude as it approached nearer the sun. On the evening 

 of January 23, 1744, it appeared exceedingly bright and distinct, and the di- 

 ameter of its nucleus was nearly equal to that of Jupiter. Its tail then extended 

 above 16 from its body, and was supposed to be about twenty-three millions of 

 miles in length. On the llth of February the nucleus, which had before been 

 always round, appeared oblong in the direction of the tail, and seemed divided 

 into two parts by a black stroke in the middle. One of the parts had a sort of 

 beard brighter than the tail : this beard was surrounded by two unequal dark 

 strokes that separated the beard from the hair of the comet. These odd phe- 

 nomena disappeared the next day, nnd nothing was seen but irregular obscure 

 spaces, like smoke, in the middle of the tail, and the head resumed its natural 

 form. On the 15th of February the tail was divided into two branches the 

 eastern, about 8 long, the western, 24. On the 23d the tail began to be bent. 

 It showed no tail till it was as near the sun as the orbit of Mars, and it in- 

 creased in length as it approached that luminary. At its greatest length it was 

 computed to equal a third part of the distance of the earth from the sun. This 

 was one of the most brilliant comets that had appeared since that of 1680. Its 

 tail was visible for a long time after its body was hid under the horizon. It 

 extended 20 or 30 degrees above the horizon two hours before sunrise. 



In the month of March, 1843, a comet appeared in the heavens exhibiting a 

 great extent of tail, but very faintly luminous. Its course was calculated from 

 the observations made upon it. but no satisfactory grounds were obtained by 

 which it might be identified with any former body of the same kind. The 

 form of the tail was remarkable, inasmuch as its edges were parallel and not 

 divergent. The length of the tail was calculated from the observations, and 

 said to amount to above one hundred millions of miles. This comet was ren- 

 dered memorable by the fact of its having passed at its perihelion so close to 

 the sun that Arago believed it must have grazed its surface. A sketch of this 

 comet is annexed on the opposite page. 



The following observations of Professor Nichol on this comet will be read 

 with interest : 



" Early in the year 1843, an object appeared in the heavens that must have 

 astonished many worlds besides ours. Situated in the region below the 

 constellation Orion, it had the appearance of a long auroral streak, visible 

 immediately after sunset, and evidently pursuing a course through our system. 

 Unfavorable weather concealed it from me until the 25th of March, when it 

 presented the dim and strange appearance I have shown in the frontispiece. The 

 beginning or head of this streak, although never observed here, was often seen 

 in southerly latitudes, where it appeared like a very small star with an enor- 

 mous misty envelope ; beyond which that immense tail streamed through the 

 sky. There is no reason to believe that this nucleus was in reality a star, but 

 only a denser portion of the nebulous substance of which the whole object was 

 composed ; for with other apparitions of the same kind, whose brighter parts 

 looked like a star, the application of a very small telescopic power has always 

 been enough to dissipate the illusion, and to resolve what seemed their solid 

 region into a thin vapor. 



' This extraordinary visiter was measured, and the nature of its path de- 

 tected ; and certainly the results of these inquiries caused us to look on it with 

 / still greater wonder. The diameter or breadth of its nucleus was rather more 

 ( than a hundred thousand miles ; and the tail streaming from it, which in some 



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