ADDRESS. xlix 
latter labour, that no fewer than seventy-seven stars, previously catalogued, 
are now missing. This, no doubt, is to be ascribed, in part, to the errors of 
former observations ; but it seems reasonable to suppose that, to some extent 
at least, it is the result of changes actually in progress in the Sidereal 
Systems. 
The sudden appearance of a new fixed star in the heavens,—its subsequent 
change of lustre,—and its final disappearance, are phenomena which have 
at all times attracted the attention of astronomers. About twenty such have 
been observed. Arago has given the history of the most remarkable, and 
discussed the various hypotheses which have been proposed for their expla- 
nation. Of these, the most plausible is that which attributes the phenomenon 
to unequal brightness of the faces of the star, which are presented success- 
ively to the earth by the star’s rotation round its axis. On this hypothesis 
the appearance should be periodic. M. Goldschmidt has recently given sup- 
port to this explanation, by rendering it probable that the new star of 1609 
is the same whose appearance was recorded in the years 393, 798, and 1203 ; 
its period, in such case, is 4054 years. 
The greater part of the celestial phenomena are comprised in the move- 
ments of the heavenly bodies, and the configurations depending on them; 
and they are for the most part reducible to the same law of gravity which 
governs the planetary motions. But there are appearances which indicate 
the operation of other forces, and which therefore demand the attention of 
the physicist,—although, from their nature, they must probably long remain 
subjects of speculation. Of these the spiriform nebulex, discovered by Lord 
Rosse, have been already referred to from this Chair, as indicating changes 
in the more distant regions of the universe, to which there is nothing entirely 
analogous in our own System. These appearances are accounted for, by an 
_ able anonymous writer, by the action of gravitating forces combined with 
the effects of a resisting medium,—the resistance being supposed to bear a 
sensible proportion to the gravitating action. 
_ The constitution of the central body of our own System presents a nearer 
and more interesting subject of speculation. Towards the close of the last 
_ century many hypotheses were advanced regarding the nature and constitu- 
q tion of the Sun, all of which agreed in considering it to be an opaque body, 
surrounded at some distance by a luminous envelope. But the only certain 
fact which has been added to science in this department is the proof given 
by Arago, that the light of the Sun emanated, not from an incandescent 
5 solid, but from a gaseous atmosphere; the light of incandescent solid bodies 
being polarized by refraction, while the light of the Sun, and that emitted by 
a gaseous bodies, is wnpolarized. 
_ According to the observations of Schwabe, which have been continued 
without intermission for more than thirty years, the magnitude of the solar 
surface obscured by spots increases and decreases periodically, the length of 
_ the period being 11 years and 40 days. This remarkable fact, and the rela- 
tion which it appears to bear to certain phenomena of terrestrial magnetism, 
1857. : d 
