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THE POPULAR SCIENCE MONTHLY. 



but separate, and following each other. Hoek, of Utrecht, showed 

 some years ago that such comet-families exist. When we compare 

 the orbits of the comets of 1843 and 1668 there is nothing that foi*bids 

 the idea of their identity. The differences are no greater than prob- 

 able perturbations might account for. Then, again, the comets of 



Fig. 4 



1843 and 1880 may easily be "identical. Indeed, the orbit given for 

 the latter comet corresponds to a period of almost thirty-seven years, 

 and Meyer has shown that the observations can not be reconciled 

 with a period less than thirty or greater than fifty years. Now, 

 thirty-seven years would take us back just to 1843, so that it is very 

 likely that these two comets are really one and the same. So far the 

 " identifiers " have matters their own way. But, now, as to the comet 

 of 1882. Can it be identical with the comet of 1880 ? We think not. 

 The orbit of the latter was computed exclusively from observations 

 taken after its perihelion passage, so that no action of the sun depend- 

 ing upon its close approach at perihelion can account for its return in 

 less than three years, and the inclination of its orbit is such that ever 

 6ince it went out of sight it has been out of harm's way as to pertur- 

 bations by the planets. Then, again, the orbit of the comet of 1882 

 does not agree with the idea of identity. Whatever other effects may 

 have been produced by the resistance of the solar atmosphere at peri- 

 helion, this resistance must have tended to shorten its period, if it 

 changed it at all. Now, the observations thus far taken, though per- 



