34 S ASTRO 
pers, the words condenfalion, clufcers, Sec. of (lars frequently 
occur, we are by no means from thence to imagine that 
any of the celeftial todies in our nebula are nearer to one 
another than we are to Sirius, whofe dhtance is fuppofed 
not to be lefs than 400,000 times that of the Sun from us, 
or thirty-eight millions of millions of miles. The whole 
extent of the nebula being in fome places near 500 times 
as great, mu ft be Inch, that the light of a ftar placed at 
its extreme boundary, itippofing it to fly with the velocity 
of twelve millions of miles every minute, mull have taken 
near 3000 years to reach us. Dr. Herfchel, however, is 
by no means of opinion, that our nebula is the mod: con- 
fiderable in the univerfe. “ As we are ufed, fays he, to 
call the appearance of the heavens, where it is {unrounded 
with a bright zone, the milky-way, it may not be ainils to 
point out fome other very remarkable nebula;, which can¬ 
not well be lefs, but are probably much larger, titan our 
•own fiyHern; and, being alfo extended, the inhabitants of 
the planets that attend the ftars which cotnpofe them 
111 lift likewile perceive the fame phenomena ; for which 
reafon they may alio be called milky-ways , by way of 
diftindtion. 
“ My opinion of their ftze is grounded on the following 
obfervations: there are many round nebulse of the firft 
.form, of about five or fix minutes in diameter, the ftars of 
which I can fee very diftintlly; and, on comparing them 
with the vifinal ray calculated from fome of my long- 
gauges, I fuppofe, by the appearance of the fmall ftars in 
Thole gauges, that the centres of thefe round nebulae may 
be 600 times the diftance of Sirius from us.” He then 
.roes on to tell us, that the ftars in fuch nebulae are proba¬ 
bly twice as much condenfed as thofe of our fiyftem; other- 
wife the centre of it would not be lefs than 6000 times the 
diftance of Sirius from us; and that it is pollible much 
under-rated by fiuppoling it only 600 times the diftance of 
that ftar. 
“ Some of thefe round nebulae, fays Dr. Herfchel, have 
others near them, perfectly (imilar in form, colour, and 
the difttribution of ftars, but of only half the diameter : and 
the ftars in them feem to be doubly crowded, and only at 
about half the diftance from each other. They are in¬ 
deed fo fmall, as not to be vifible without the utmoft at¬ 
tention. 1 fuppofe thefe miniature nebulte to be at double 
the diftance of the firft. An inftance equally remarkable 
and inftruftive is a cafe where, in the neighbourhood of 
two Inch nebulae as have been mentioned, I met with a 
third, fimilar, refolvable, but much fmaller and fainter, 
nebula. The ftars of it are no longer to be perceived ; 
but a refemblance of colour with the former two, and its 
diminiflied ftze and light, may well permit us-to place it 
at full twice the diftance of the fecond, or about four or 
five times the .diftance of the firft. And yet the nebulo- 
tity is not of the milky kind ; nor is it fo much as difficult¬ 
ly refolvable or colourlefs. Now in a few of the extended 
nebulae, the light changes^gradually, fo as from the re¬ 
folvable to approach to the'milky kind; which appears to 
me an indication, that the milky light of nebula: is owing 
to" their much greater diftance'. A nebula, therefore, 
whole light is perfe&ly milky, cannot well be fuppofed to 
be at left than fix or eight thoufand times the diftance of 
Sirius ; and, though the numbers here affumed are not to 
be taken otherwiie than as very coarfe eftiniates, yet an 
extended nebula, which an oblique lituation, where it is 
poftibly fore-fihortened by one-half, two-thirds, or three- 
fourths, of its length, iubtends a degree or more in dia¬ 
meter, cannot be other wife than of a wonderful magnitude, 
and well may outvie our milky-way in grandeur. -And 
what great length of time mult be required to produce the 
formation of nebulae, may eafiiy be conceived, when, in 
all probability, our whole fiyftem, of about 800 ftars in 
diameter, if it were feen at fuch a diftance that one end 
of it might affume die refolvable nebulofity, would not, 
at the other end, prefent us with the irrefolvable, much 
lefs with the colourlefs and milky, fort of nebulofities.” 
.Qreat indeed mult be the length of time requifite for fuch 
N O M Y. 
diftant bodies to form combinations by the laws of attrac¬ 
tion, fince, according to the diftances aflumed, the light 
of fome of the nebulae muft be thirty-fix or forty-eight 
thoufand years in arriving from them to us. It would be 
worth white then to enquire, whether attraction is a virtue 
propagated in time, or not; or whether it moves quicker 
or flower than light ? 
In the courfe of Dr. Herfchel’s obfervations, an objec¬ 
tion occurred, that, if the different fyftems were formed 
by the mutual attractions of the ftars, the whole would be 
in danger of deftruction by the falling of them one upon 
another. A fufficient anfvver to this, he thinks, is, that, 
if we can really prove the fyftem of the univerfe to be 
what he has laid, there is no doubt but that the great 
Author of it has amply provided for the prefervation of 
the whole, though it Ihould not appear to us in what 
manner this is effected. Several circumftances, however, 
he is of opinion, manifeftly tend to a general prefervation; 
as, in the firft place, the indefinite extent of the lidercat 
heavens ; which muft produce a balance that will effectu¬ 
ally fecure all the great parts of the whole from approach¬ 
ing to each other. “ There remains then, fays he, only 
to fee how the particular ftars belonging to feparate cluf- 
ters are prevented from ruftiing on to their centres of at¬ 
traction.” This he fuppofes may be done by projeCtde 
forces; “ the admiffion of winch will prove fuch a barrier 
againft the feeming deftruCtive power of attraction, as to 
fecure it from all the ftars belonging to a clufter, if not 
for ever, at lead for millions of ages. Befides, we ought 
perhaps to look upon fuch clulters, and the deftruCtion of- 
a ftar now and then in fome thoufands of ages, as the very 
means by which the whole is preferved and renewed. 
Thefe clulters may be the laboratories of the univerfe, 
wherein the moll falutary remedies for the decay of the 
whole are prepared.” 
Purfuing ftill farther this inveftigation of the' fidereal 
heavens, Dr. Herfchel has difeovered other phenomena, 
which he calls nebulous ftars ; that is, Jingle ftars, furround- 
ed with a lucid atmofphere of confiderable extent. He has 
given an account of feventeen of thefe ftars, one of which 
he has thus deferibed. “ November 13, 1790, A moft 
Angular phenomenon! A ftar of the eighth magnitude, with 
a faint luminous atmofphere, of a circular form, and of 
about three minutes diameter. The ftar is perfectly in 
the centre, and the atmofphere is fo diluted, faint, and 
equal throughout, that there can be no fiurmife of its con¬ 
fiding of ftars ; nor can there be a doubt of the evident 
connection between the atmofphere and the ftar. Ano¬ 
ther ftar not much lefs in brightnefs, and in the fame field 
of view with the above, was perfectly free from any fuch 
appearance.” Hence he draws the following confequences. 
Granting the connection between the ftar and the furround¬ 
ing nebulofity, if it conlift of ftars very remote which gives 
the nebulous appearance, the central ftar, which is vifible, 
muft be immenfely greater than the reft ; or if the central 
ftar be no bigger than common, how extremely fmall and 
compreffed muft be thofe other luminous points whichoc- 
cafion the nebulofity? As, by the former fuppolition, the 
luminous central point muft far exceed the ftandard of what 
we call a ftar, fo, in the latter, the fhining matter about 
the centre will be much too fmall to come under the fame 
denomination; we therefore either- have a central body 
which is not a ftar, or a ftar which is involved in a Ihining 
fluid, of a nature totally unknown to us. This lafit opi¬ 
nion Dr. Herfchel adopts. The exiftence of this fhining 
matter, he fays, does not feem to be fo effentially connected 
with the central. points, that it might not exift without 
them. The great refemblance there is between the cheve- 
lure of thefe ftars, and the dift'tifed nebulofity about the 
conftellation of Orion, which takes up a fpace of more 
than lixty fquare degrees, renders it highly probable that 
they are of the fame nature. If this be admitted, the fe¬ 
parate exiftence of the luminous matter is fully proved. 
Light retleCled from the ftar could not be feen at this dif¬ 
tance ; and befides, the outward parts are nearly as bright 
as 
