423 ASTRO 
feCfual, way, to exp refs the brightncfs of a ftar, and by 
that means to deled any change that may happen in its 
luftre, will appear, when we conlider what is requifite to 
afcertain fucli a change. We can certainly not with for a 
Aiiore decifive evidence, than to be affined by aCtual in- 
fpeCtion, that a certain fear is now no longer more or lefs 
bright than f’uch other tlars to which it lias been formerly 
compared ; provided we are at the fame time atfnred, that 
thofe other ftars remain ftill in their former unaltered luf- 
tre. But, if the ftar D will no longer ftana in its former 
order C D E, it mud have undergone a change; and, if 
that order is now to be exprefled by C E D, the (tar has 
loft forae part of its luftre ; if, on the contrary, it ought 
now to be denoted by D C E, its brightnefs mud have 
had fome addition. Then, if we (hould (jlbubt the (labi¬ 
lity of C and E, we have recourfe to the orders BCD 
'and D E F, which exprefs their luftre ; or even to A B C 
and E F G, which continue the feries both ways. Now, 
having before us the feries B C D E F, or if necelfary even 
the more extended one A B C D E F G, it will be impof- 
(ible to mi (take a change of brightnefs in D, when every 
member of the feries is found in its proper order except D.” 
In Dr. Herfchel’s catalogue, therefore, in which the or¬ 
der of the luftre of the (tars is fixed, each (tar bears its 
own proper name or number, e.. g. “ the brightnefs of the 
(tar £Leonis may be exprelfed by @ o e Leonis, or better by 
94—68—17 Leonis; thefe being the numbers which the 
three (tars bear in the Britifii Catalogue of fixed (tars.” 
Dr. Herfchel adopted this method of arrangement about 
fourteen years ago ; but he was diverted from the regular 
purfuit of it by a variety of other aftronomical engage¬ 
ments. After many trials, the plan which appeared to 
him the mod eligible was as follows : Inftead of denoting 
particular (tars by letters, he makes ufettf numbers; and, 
in his choice of the (tars which are to exprefs the luftre of 
any particular one, he directs his firft view to perfect equa¬ 
lity. When two (tars feem to be fimilar, both in bright¬ 
nefs and magnitude, he puts down their numbers together, 
feparated merely by a point, as 30.24 Leonis: but, if two 
(tars, which at firft teemed alike in their luftre, appeared 
on a longer infpeftion to be different, and the preference 
(hould be always decidedly in favour of the fame (tar, he 
feparates thefe (tars by a comma, thus, 41,94 Leonis. This 
order nuift not be varied ; nor can three fucli (tars as 20, 
40, 39, Librae, admit of a different arrangement. If the 
ftate of the heavens (hould be fuch as to require a diffe¬ 
rent order in thefe numbers, we may certainly infer, that 
a change has taken place in the luftre of one or more of 
them. " When two (tars differ very little in brightnefs, 
but fo that the preference of the one to the other is indif- 
putable, the numbers that exprefs them are feparated by 
a ftiort line, as 17—70 Leonis, or 68 — 17—70 Leonis. 
When two ftars differ fo much in brightnefs, that one or 
two other ftars might be interpofed between them, and 
ftill leave fufficient room for diftinCtion, they are diftin- 
guiftied by a line and comma, thus —, or by two lines, as 
32-41 Leonis. A greater difference than this is deno¬ 
ted by a broken line, thus 16-29 Bootis. On the 
whole, Dr. Herfchel obferves, the marks and diftinCtions 
which he has adopted cannot polTibly be miftaken; “a 
point denoting equality of luftre ; a comma indicating the 
lead perceptible difference ; a ftiort line to mark a decided 
but fmaU fuperiority; a line and comma, or double line, 
to exprefs a. confiderable and ftriking excefs of brightnefs; 
and a broken line to mark any other fuperiority which is to 
be looked upon as of no ufe in eftimations that are intend¬ 
ed for the purpofe of directing changes.” 
The difficulties that attend this arrangement are not dif- 
guifed : but the importance and utility of it more than 
compenfate for the labour which it muff neceffarily re¬ 
quire, By a method of this kind, many dil'coveries of 
changeable and periodical ftars might probably have been 
made, which have elcaped the raoff diligent and accurate 
obfervers. We might then, as the author fuggefts, be 
enabled to refolve a problem in which we are all imrne- 
N O M Y. 
diately concerned. “ Who (fays Dr. Herfchel) would not 
wifti to know what degree of permanency we ought to 
aferibe to the luftre of our Sun. ? N> t only the (lability 
of our climates, but the very exiftence of the whole ani¬ 
mal and vegetable creation itfelf, is involved in the quef- 
tion. Where can w r e Hope to receive information upon 
this fubjeCi but from aftronomical obfervations ? If it be 
allowed to admit the fimilarity of ftars with our Sun as a 
point edab)tilled, how necelfary will it be to take notice of 
the fate of our neighbouring funs, in order to guefs at that 
of our own! The (far, which among the multitude we- 
have dignified by the name of fun, to-morrow may fiowly 
begin to undergo a gradual decay of brightnefs, like @ 
Leonis, a Ceti, aDraconis, ^Urfas Majoris, and many other 
diminiihing ftars, that will be mentioned in my catalogues. 
It may fuddenly increafe, like the wonderful ftar in the 
back of Caftiopea’s chair, and the no lefs remarkable one 
in the foot of Serpentarius ; or gradually come on like 
/3 Geminorum, /3 Ceti, £ Sagittarii, and many other in- 
creafing ftars, for which I alfo refer to my catalogues; 
and laftly, it may turn into a periodical one of twenty-five 
days duration, as Algol is one of three days, JCephei of 
five, /3 Lyrae of fix, n Antinoi of feven days, and as many 
others as are of various periods.” 
As many phenomena in natural hiftory feem to point out 
fome pall changes in our climates, the eafieft way of ac¬ 
counting for them may probably be ‘ to furmife that our 
Sun has been formerly fometimes more and fometimes lefs 
bright than it is at prefent; at all events, it will be highly 
prefumptuous to lay any great ftrefs upon the (lability of 
the prefent order of things ; and many hitherto unaccount¬ 
able varieties that happen in our feafons, fuch as the ge¬ 
neral feverity or mildnefs of uncommon winters or burn¬ 
ing fummers, may poftibly meet with an eafy folution in 
the real inequality of the Sun’s rays.’ 
The utility of the above method of eftimating the com¬ 
parative brightnefs of the ftars, Dr. Herfchel has in a 
great meafnre verified by experience. He. has very lately 
dificovered a Herculis to be a periodical ftar. By a feries 
of obfervations on this ftar, compared with v. Ophiuchi, 
w hich was molt conveniently (ituaied for his purpofe, he 
lias been able not only to confirm this opinion, but to af¬ 
certain its period. His obfervations are arranged in a ta¬ 
ble, by means of which he determines that this ftar lias 
gone through four fucceftive changes in an interval of 241 
days; and therefore the duration of its period mud be 
aboift fixty days and a quarter. This faCt concurs with 
other circumftances in evincing the rotatory motion of the 
ftars on their axes. “ Dark fpots, (he fays,) or large por¬ 
tions of the furface, lefs luminous than the reft, turned 
alternately in certain directions, either towards or from us, 
will account for all the phenomena of periodical changes 
in the luftre of the ftars, fo fatisfaclorily, that we certain¬ 
ly need not look out for any other caufe.” If it be al¬ 
leged, that the periods in the change of luftre of fome 
ftars, fuch as Algol, /3 Lyras, oCephei, and u Antinoi, are 
(hort, being only 3, 5, 6, and 7, days refpeCtively; while 
thofe of 0 Ceti, of the changeable ftar in Hydra, and that 
in the neck of the Swan, are long, amounting to 331, 394, 
and 497, days; and that we cannot aferibe phenomena fo 
different in their duration to the fame caufe; it may be 
anfwered to this objection, that the force of it is founded 
on our limited acquaintance with the ftate of the heavens. 
To the feven ftars, the periodical changes of which were 
before known, we may now add a Herculis, which performs 
a revolution of its changes in fixty days. ‘ The ftep from 
the rotation of a. Herculis to that of 0 Ceti is far lefs con¬ 
fiderable than that from the period of Algol to the rota¬ 
tion of a, Herculis; and thus a link in flip chain is now 
funplied, which removes the objection that arofe from the 
vacancy.’ The rotation of the fifth fatellite of Saturn is 
proved by the change obfervable in its light; and ‘ this 
variation of light, owing to the alternate expofition of a 
more or lefs bright hemifphere of this periodical fatellite, 
plainly indicates that the fimilar phenomenon of a change. 
able 
