SECT. XXXV. MASSES OF COMETS. 373 



of its tail continued to increase till the latter stretched far "be- 

 yond the constellation of the Hare towards a point above Sirins. 

 Stars were distinctly seen through it, and when near perihelion 

 the comet was so bright that it was seen in clear sunshine, in 

 the United States, like a white cloud. The motion was retro- 

 grade, and on leaving the solar system it retreated so rapidly at 

 once from the sun and earth that it was soon lost sight of for 

 want of light. On the 1st of April it was between the sun and the 

 earth, and only 40 millions of miles from the latter ; and as its 

 tail was at least 60 millions of miles long, and 20 millions of 

 miles broad, we probably passed through it without being aware 

 of it. There is some discrepancy in the different computations 

 of the elements of the orbit, but in the greater number of cases 

 the perihelion distance was found to be less than the semidiameter 

 of the sun, so that the comet must have grazed his surface, if it 

 did not actually impinge obliquely on him. 



The perihelion distance of this comet differs little from that 

 of the great comet of 1668, which came so near the sun. The 

 motion of both was retrograde, and a certain resemblance in the 

 two orbits makes it probable that they are the same body per- 

 forming a revolution in 175 years. 



Though already so well acquainted with the motions of comets, 

 we know nothing of their physical constitution. A vast number, 

 especially of telescopic comets, are only like clouds or masses of 

 vapour, often without tails. The head commonly consists of 

 a concentrated mass of light, like a planet, surrounded by a 

 very transparent atmosphere, and the whole, viewed with a 

 telescope, is so diaphanous, that the smallest star may be seen 

 even through the densest part of the nucleus ; in general their 

 solid parts, when they have any, are so minute, that they have no 

 sensible diameter, like that of the comet of 1811, which appeared 

 to Sir William Herschel like a luminous point in the middle of 

 the nebulous matter. The nuclei, which seem to be formed of 

 the denser strata of that nebulous matter in successive coat- 

 ings, are sometimes of great magnitude. Those comets which 

 came to the sun in the years 1799 and 1807 had nuclei whose 

 diameters measured 180 and 275 leagues respectively, and the 

 second comet of 1811 had a nucleus 1350 leagues in diameter. 



It must, however, be stated that, as comets are generally at 

 prodigious distances from the earth, the solid parts of the nuclei 



