SECT, xxxvi.] HALLEY'S COMET. 391 



system, or in the regions beyond it, the comet has appeared 

 exactly at the time, and not far from the place, assigned to 

 it by astronomers ; and its actual arrival at its perihelion a 

 little before noon on the 16th of November, 1835, only 

 differed from the computed time by a very few days, which 

 was probably owing to the attraction of Neptune. 



The fulfilment of this astronomical prediction is truly 

 wonderful if it be considered that the comet is seen only for 

 a few weeks, during its passage through our system, and 

 that it wanders from the sun for seventy-five years to twice 

 the distance of Uranus. This enormous orbit is four times 

 longer than it is broad ; its length is about 3420 millions of 

 miles, or about thirty-six times the mean distance of the 

 earth from the sun. At its perihelion the comet comes 

 within nearly fifty-seven millions of miles of the sun, and 

 at its aphelion it is sixty times more distant. On account 

 of this extensive range it must experience 3600 times more 

 light and heat when nearest to the sun than in the most 

 remote point of its orbit. In the one position the sun will 

 seem to be four times larger than he appears to us, and at 

 the other he will not be apparently larger than a star 

 (N. 221). 



On the first appearance of Halley's comet, early in 

 August 1835, it seemed to be merely a globular mass of 

 dim vapour, without a tail. A concentration of light, a 

 little on one side of the centre, increased as the comet ap- 

 proached the sun and earth, and latterly looked so like the 

 disc of a small planet, that it might have been mistaken 

 for a solid nucleus. M. Struve, however, saw a central 

 occultation of a star of the ninth magnitude by the comet, 

 at Dorpat, on the 29th of September. The star remained 

 constantly visible, without any considerable diminution of 

 light ; and, instead of being eclipsed, the nucleus of the 

 comet disappeared at the moment of conjunction from the 

 brilliancy of the star. The tail increased as the comet 

 approached its perihelion, and, shortly before it was lost in 



