35° 
ASTRONOMY. 
srets, perhaps unattended by fatellites.” After a variety 
of fimilar remarks, and many ingenious obfervations on 
the probable deftination of two thoufand diftant nebula 
and clufters of ftars difcovered by himfelf, Dr. Herfchel 
concludes tints : “ Upon the whole, 1 believe, it will be 
found, that the foregoing theoretical view, confidered all 
together, with its conlequential appearances, is no other 
than a drawing from nature, wherein the features of the 
original have been clofely copied ; and I hope the refeni- 
hlance will not be called a bad one, when it fhall be con¬ 
fidered how very limited mult be the pencil of an inhabi¬ 
tant of fo fmall and retired a portion of an infinite lyftem, 
in attempting the picture of fo unbounded an extent.” 
Plate II. of Aftronomv will furnilh fome idea of this uni- 
verfal fyftem of the fidereal heavens, and of the nebulae 
and lucid ftars; throughout the whole of which there is 
an evident tendency towards fpjnericity/by the fwell of the 
dimenlions as they draw near the niort luminous place, 
denoting, as it were, a courfe, or tide of ftars, fetting to¬ 
wards a centre. 
Of ike BODIES which FORM the SOLAR SYSTEM. 
Of the SUN. 
The Sun, which was reckoned among the planets in the 
infancy of aftronomy, Ihould rather be counted among the 
fixed (tars. He only appears brighter and larger than they 
do, becaufe we keep conftantly near him ; whereas we are 
immenfely farther from the ftars. But a fpedfator, placed 
as near to any ltar as we are to the Sun, would probably 
fee that ftar a body as large and as bright as the Sun ap¬ 
pears to us ; and, on the other hand, a fpeblatoras far dif- 
tant from the Sun as we are from the ftars, would fee the 
Sun as fmall as we fee a ftar, divefted of all his circum- 
volving planets ; and he would reckon it as one of the 
ftars, in numbering them. 
The opinions of the ancients concerning the nature and 
properties of the Sun, were extremely imperfect and ob- 
icure. Anaximander and Anaximenes held, that there 
was a circle of fire extending throughout the heavens, 
called the circle cf the Sun ; between the earth and this 
fiery circle they imagined another circle of an opaque fub- 
ftance, in which was a hole like the mouth of a German 
flute. Through this hole they fuppofed the light to be 
tranfmitted ; w hence it appeared to the inhabitants of this 
earth as a round and diftinft body of fire ; and eclipfes of 
the Sun, they thought, were occaiioned by the Hopping of 
this hole. Anaxagoras, however, and mod of the ancient 
afironomers, held the Sun to be a globe of fire, of fome 
folid fubftance ; and many of the moderns have adopted 
the fame opinion. Thofe who have maintained that the 
fubftance of the Sun is fire, argue in the following man¬ 
ner : the Sun (bines, and his rays, collected by concave 
mirrors, or convex lenfes, burn, confume, and melt, the 
mod folid bodies, or elfe convert them into allies, or glafs : 
therefore, as the force of the folar rays is diminifhed, by 
their divergency, in a duplicate ratio of the diftances reci¬ 
procally taken; it is evident that their force and effedt are 
the fame, when collected by a burning lens, or mirror, 
as if we were at fuch diftance from the Sun, where they 
were equally denfe. The Sun’s rays therefore, in the 
neighbourhood of the Sun, produce the fame effedts, as 
might be expedted from tlie moft vehement fire; confe- 
quently the Sun is of a fiery fubftance. Hence it follows, 
that its furface is probably every where fluid; that being 
the condition of flame. Indeed, whether the whole body 
cf the Siui be fluid, as fome think, or folid, as others, they 
do not'prefume to determine ; but as there are no. other 
marks, by which to diftinguifh fire from other bodies, but 
light, heat, a power of burning, confuming, melting, cal¬ 
cining, and vitrifying; they do not fee what fliould hinder 
but that the Sun may be a globe of fire, like our fires, in¬ 
verted with flame ; and, fuppofing that maculae are formed 
out of the folar exhalations, tlrey infer that the Sun is not 
pure fire, but that there are heterogeneous parts mixed 
along with it. Sir Ifaac Newton has propofed it as aquery 5 
Whether the Sun and fixed ftars are not great earths made 
vehemently hot, whofe parts are kept from fuming away 
by the vaft weight and denfity of their fuperincumbent at- 
mofpheres, and whofe heat is preferved by the prodigious 
action and readtion of their parts upon one another ? 
Philofophers have been much divided alfo in their opi¬ 
nion with refpedt to the nature of fire, light, and heat, and 
the caufes that produce them : and they have given very 
different accounts of the agency of the Sun, with which*, 
whether we confider them as fubftances or qualities, they 
are intimately connedted, and on which they feem primal 
rily to depend. Some, among whom we may reckon Sir 
Ifaac Newton, conlider the rays of light as compofed of 
fmall particles, which are emitted from fhining bodies, 
and move with uniform velocities in uniform mediums, 
but with variable velocities in mediums of variable denfi- 
ties. Thefe particles, fay they, adt upon the minute con- 
ftituent parts of bodies, not by irnpadt, but at fome inde¬ 
finitely fmall diftance; they attradl and are attracted ; and, 
in being refledted or refracted, they excite a vibratory mo¬ 
tion in the component particles. This motion increafes 
the diftance between the particles, and thus occafions an 
augmentation of bulkf or an expanfion in every dimen. 
fion, which is the moft certain charadteriftic of fire. This 
expanfion, which is the beginning of a difunion of the 
parts, being increafed by the increafing magnitude of the 
vibrations proceeding from the continued agency of light, 
it may ealily be apprehended, that the particles will at 
length vibrate beyond their fphere of mutual attraction, 
and thus the texture of the body will be altered or deltroy- 
ed; from folid it may become fluid, as in melted gold; 
or, from being fluid, it may be difperfed in vapour, as in 
boiling water. 
Others, as Boerhaave, reprefent fire as a fubftance fui 
generis, unalterable in its nature, and incapable of being 
produced or deftroyed ; naturally exifting in equal quan¬ 
tities in all places, imperceptible to our fenfes, and only 
difcoverable by its effedts, when, by various caufes, it is 
colledted for a time into a lefs fpace than that which itr 
would otherwife occupy. The matter of this fire is not in 
any wife fuppofed to be derived from the Sun: the folar 
rays, whether diredt or refledted, are of ufe only as they 
impel the particles of fire in parallel directions : that pa- 
rallelifm being deftroyed, by intercepting the folar rays, 
the fire inftantly affumes its natural (late of uniform diffu- 
fion. According to this explication, which attributes heat 
to the matter of fire, when driven in parallel directions, a 
much greater degree mult be given it when the quantity, 
fo collected, is amaffed into a focus ; and yet the focus of 
tlie largeft Ipeculum does not heat the air or medium in 
which it is found, but only bodies of denfities different 
from that medium. 
M. de Luc, in his Lettres Phyfiques, is of opinion, that 
the folar rays are the principal caufe of heat; but that they 
heat fuch bodies only as do nqt allow them a freepaflage. 
In this remark he agrees with Sir Ifaac Newton ; but then 
lie differs totally from him, as well as from Boerhaave, 
concerning the nature of the rays of the Sun. He does 
not admit the emanation of any luminous corpufcles from 
the Sun; or other lelf-fliining fubftances, but fuppofes all 
fpace to be filled with an ether of great elafticity and fmall 
denfity, and that light conlifts in the vibrations of this 
ether, as found confifts in the vibrations of the air. “ Up¬ 
on Newton’s fuppofition, fays an excellent writer, the caufe 
by which the particles of light, and the corpufcles confti- 
tuting other bodies, are mutually attracted and repelled, is 
uncertain. The reafon of the uniform diffufion of fire, of 
its vibration, and repercuflion, as ftated in Boerhaave’s 
opinion, is equally inexplicable. And in the laft-men- 
tioned hypothefis, we may add to the other difficulties 
attending the fuppofition of an univerfal ether, the want 
of a firft mover to make the Sun vibrate. Of thefe feve- 
ral opinions concerning elementary fire, it may be faid, as 
Cicero remarked upon tlie opinions of philofophers con¬ 
cerning 
