COMETS. 



between the centres of the two bodies would have been only 

 18600 miles, while the semi- diameter of the comet was 21000 

 miles ; and the semi-diameter of the earth being in round 

 numbers 4000 miles, it would follow that in such a contingency 

 the earth would have plunged into the comet to a depth of 



21000 + 4000 18600 = 6400 miles, 



a depth exceeding three-fourths of the earth's diameter. 



The possibility of such a catastrophe having been rumoured, 

 great popular alarm, was excited before the expected return of the 

 comet in 1832. It was, however, shown that on the 29th October 

 the earth would be about five millions of miles from the point of 

 danger, and that, on the arrival of the earth at that point, the 

 comet would have moved to a still greater distance. 



28. Resolution of Biela's comet into two. One of the most 

 extraordinary phenomena of which the history of astronomy 

 affords any example, attended the appearance of this comet in 

 1846. It was on that occasion seen to resolve itself into two 

 distinct comets, which, from the latter end of December, 1845, to 

 the epoch of its disappearance in April, 1846, moved in distinct 

 and independent orbits. The paths of these two bodies were 

 in such optical juxtaposition that both were always seen together 

 in the field of view of the telescope, and the greatest visual angle 

 between their centres did not amount at any time to 10', the 

 variation of that angle arising principally from the change of 

 direction of the visual line, relatively to the line joining their 

 centres, and to the change of the comet's distance from the earth. 

 M. Plantamour, director of the Observatory of Geneva, calcu- 

 lated the orbits of these two comets, considered as independent 

 bodies ; and found that the real distance between their centres 

 was, subject to but little variation while visible, about thirty- 

 nine semi-diameters of the earth, or two-thirds of the moon's 

 distance. The comets moved on thus side by side, without 

 manifesting any reciprocal disturbing action ; a circumstance 

 no way surprising, considering the infinitely minute masses of 

 such bodies. 



29. The original comet was apparently a globular mass of 

 nebulous matter, semi-transparent at its very centre, no appear- 

 ance of a tail being discoverable. After the separation, both 

 cornets had short tails, parallel in their direction, and at right 

 angles to the line joining their centres ; both had nuclei. From 

 the day of their separation the original comet decreased, and the 

 companion increased, in brightness, until (on the 10th February) 

 they were sensibly equal. After this the companion still increased 

 164 



