402 MASSES OF COMETS. [SECT, xxxvi. 



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 resem- 

 blance in the two orbits makes it probable that they are the 

 same body performing 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. Such were 

 the comets which appeared in the years 1795, 1797, and 

 1798. But 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, 

 if they have any, are so minnte, 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 suc- 

 cessive coatings, 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 



