1888.] President''s Address. xxxix 



between the sun and these vapours. Breclichin's division of the tails 

 into three types corresponding to hydrogen, hydro-carbons, and 

 metallic vapour respectively has been amply verified (at least as 

 regards the two first). But perhaps the most important fact which 

 has been brought to light is that there undoubtedly exist families 

 of comets following almost exactly the same path in space. Formerly 

 identity of path was considered conclusive evidence of identity in 

 the objects, but we can no longer say this. Tebbutt's comet of 

 1881 was moving in almost exactly the same path as the comet of 

 1807 for which Bessel in a classical discussion had deduced a period 

 of 1,539 years. Fortunately Tebbutt's comet was visible for a long 

 time and the observations of it proved that it was moving in an orbit 

 of about 3,000 years period, so that it was not a return of the 1807 

 one. Again the Southern comet of 1880 followed a path almost 

 identical with that of the great 1843 comet which was visible in 

 broad daylight near the sun ; and the great comet of 1882 and the 

 first of 1887 were also moving in this same orbit — yet they are all 

 distinct bodies though undoubtedly members of the same family. It 

 mast be remembered that the comets in this group pass close to the 

 sun's surface and right through the corona ; and though observation 

 shewed that the 1882 comet had not suffered the slightest retardation 

 in doing so, (thus giving the final blow to Encke's resisting medium), 

 yet. several faint nebulous masses were observed after perihelion 

 travelling along with the comet and at some little distance from it, 

 which had evidently been torn away from the main body. Thus it 

 is not difficult to imagine that the comets of '43, '80 and '87 were 

 separated from the original comet at some previous passage through 

 perihelion ; and indeed it seems to me that we may go further and 

 conjecture that the '43 comet was separated from the parent body 

 (the remains of which I take to be the '82 one) at an earlier approach 

 to the sun than the '80 and '87 comets, partly on account of its much 

 greater brilliancy and partly because its tail belongs to Bredichin's 

 type I, corresponding to hydrogen, the lightest known element, while 

 that of the '80 comet belongs to type Tl, the hydro-carbons. An 

 interesting and instructive observation, unique in the history of 

 astronomy, was made by two observers at the Cape in 1882: the 

 *' Great September Comet " was followed in the telescope till it reached 

 the sun's limb, when it disappeared completely though it was passing 

 in front of the sun's disc. Shortly before the transit it shewed as a 

 brilliant circular disc 4" in diameter, but the disappearance proved 



