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PHYSICAL CONSTITUTION OF COMETS. 



493 



into two, one immediately encompassing the nucleus, the other of a moro faint 

 and grayish light, sweeping round it at a distance and forming its double tail. 

 The head-veil, as he called it, surrounded the nucleus at a distance equal to its 

 breadth, and seemed as unconnected with the nucleus as the ring of Saturn is 

 with its body. The diameter of this ring measured nearly a million of milrs, 

 being greater than the diameter of the sun. Between the 4th and Gth of De- 

 cember a great change took place in its appearance, the rarefied nebulous mat- 

 ter, which had for three months been so unusually repelled from the nucleus 

 on every side, was again attracted to it. 



The double tail of this comet was exceedingly faint when compared with its 

 nucleus. On the 16th of October a small tail instantaneously issued from it, 

 then vanished, and suddenly reappeared, when its length was nearly two mill- 

 ions and a half of miles. 



Herschel's estimate of the magnitude of the nucleus is much less than that 

 of Schrtieter ; he calculates that, on the 15th of October, the tail measured one 

 hundred millions of miles, and was, consequently, greater than the entire dis- 

 tance of the sun from the earth. He estimated its breadth on the 12th of Octo- 

 ber at fifteen millions of miles. 



Attempts have been made to calculate on probable grounds the elliptic orbit 

 of this cornet. Bessel computed that its period is three thousand three hundred 

 and eighty-three years, and other astronomers make it more than four thousand 

 years. A sketch of the comet of 1811 is annexed. 



The comet of 1680 exhibited a tail measuring 68, of a curved form ; of 

 which a traditional sketch is annexed. 



The comet of 1680, which was observed by all the European astronomers 

 of that day, exhibited a tail which extended over 90 of the heavens at its peri- 

 helion ; its distance from the surface of the sun was not more than one sixth 

 of the sun's diameter ; and it was calculated in that position to have a velocity 

 of more than 120,000 miles an hour. When the head of this comet was seen 

 at the zenith, its tail reached the horizon. The actual length of the tail was 

 calculated to be one hundred and twenty-three millions of miles ; so that if the 

 head of this comet were at the sun, the tail would extend thirty millions of 

 miles beyond the earth's orbit. 



In 1769 a comet appeared, the tail of which spread over a space of 97 

 of the heavens, and its actual length was fifty millions of miles. Difier- 

 ent estimates have been given of the length of the tails of the comet of 1744. 



