ON COMETS. 123 



(38.) The number of comets whose periodical return 

 has been calculated is pretty considerable. Altogether 

 about 36 ; and of these there are 5 which revolve in 

 periods of from 70 to So years, and several of the rest in 

 short periods from 3 to 7 years ; and it is a very remark- 

 able feature in their history that all the comets of short 

 period, and three out of the five of those of the larger 

 ones specified, revolve in the same direction round the 

 sun as the planets, and have their orbits inclined at no 

 very large angles to the ecliptic. 



(39.) Of comets not periodical, I have already men- 

 tioned that most remarkable one of 1680, but several 

 others deserve special notice. That of 1744 was. a truly 

 wonderful object. It is described, and has been depicted, 

 with six tails spread out like an immense fan extending 

 30 from the head which is fully the extent of the tail of 

 the comet of 1858; and the appearance of its head when 

 viewed through a telescope exhibited the same sort of 

 jets of luminous smoke, the same curved envelopes and 

 arches as I have already described, showing the same 

 kind of excitement by the sun's heat, and the same 

 action driving the vapour back into the tail. 



(40.) The comet of 1843 was still more remarkable. 

 Many of my hearers, I dare say, remember its immense 



been conspicuously visible but without success ! giving rise to the 

 strangest theories. At all events it seems to have fairly disappeared, 

 and that without any such excuse as in the case of Lexell's, the pre- 

 ponderant attraction of some great planet. Can it have come into 

 contact or exceedingly close approach to some asteroid as yet undis- 

 covered ; or, peradventure, plunged into and got bewildered among 

 the ring of meteorolites, which astronomers more than suspect ? 



