COMETS. 



Ill 



observed comets, 

 follows : 



January 14 

 February 10 

 March 8 



Thus, the number accomplishing the perihelion in different months is as 



April 



May 



June 



10 



9 



11 



July 10 



August 8 



September 15 



October 1 1 

 November 1 8 

 December 13 



137 



The motion of 69 was direct, or from west to east, and of 68 retrograde, or from east 

 to west. The annexed table shows the inclination of their paths to the plane of the earth's 

 orbit, from which, it should be remembered, the great planets very slightly vary : 



9 comets 



13 

 10 

 17 



14 

 23 



Inclination. 



0tO 10 

 - 10 20 

 . 20 30 

 . fjQ 40 

 . 40 50 

 . 50 60 



17 comets 

 19 

 15 



137 



Inclination. 

 - 60 to 70 

 . 7080 

 _ 80 9O 



The periodic time of these comets has been calculated, and supposed to be in the follow- 



ing cases : 



Year of Comet. 

 1807 

 1811 



1815 



1819 second comet 



1819 fourtb comet 



1812 



1 822 second comet 



1822 third comet 



1825 fourth comet 



Period. 

 1543 years 

 . 3383 



- 2888 

 . 7277 



5J - 



Sf - 



.66-76 



- 1550 



- 1817 



556 



Authority. 



Bessel. 



Bessel. 



Argelander 



Bessel and others. 



Encke. 



Encke. 



Encke. 



Encke. 



Rumker. 



Hansen. 



These predictions remain to be verified. In three cases only has the periodic time been 

 established by the return of the body that of Encke's comet whose period is 3^ years ; 

 Biela's, 6J ; and Halley's, 75^. 



The three features of nebulosity, nucleus, and tail, are usually assigned to cometary 

 bodies, but many are destitute of the latter appendage, and also without any clearly 

 defined nucleus. Tlrey appear as simple nebulosities, globular masses of vapour, having 

 no central condensation, through which the feeblest of the stars readily shine. Herschel 

 perceived a star of the sixth magnitude through the centre of the comet without nucleus 

 of the year 1795; and a star of the eleventh magnitude was perfectly distinguished by 

 Struve through the middle of one of the short-period comets. Others present a nucleus 

 strongly defined, with surrounding nebulosity, the " horrid hair " of poetry. The 

 vapoury envelope is dim towards the central point, but suddenly becomes luminous 

 at some distance from it, so as to resemble a ring resting in equilibrium around a star, 

 like the ring of Saturn. The cometary nuclei often shine with a light as vivacious as 

 that of the planets, and exceed them in splendour upon nearing the sun. They vary 

 considerably in their diameters, but are in general very small. The measurement of the 

 diameters of five given by Arago range between thirty-three miles and three thousand 

 two hundred. The external appearance of other comets exhibits the three features 

 combined, and these are remarkable objects, occasionally presenting a terrific aspect. 

 Immense spaces are sometimes covered by the luminous trains, or tails, as much as ninety 

 or a hundred degrees ; so that while the nucleus has been below the horizon, the train has 

 reached the zenith, stretching through an extent of nearly a hundred and fifty millions of 



