P L A 
P L A 
operates; that the particular aptitude of any body to be 
heated is nearly as the elafticity of that body, or the co- 
hefion of its parts; that whatever produces a tremulous 
motion in the particles of any body, excites heat in that 
body; and, vice verfa, whatever excites heat produces a 
tremulous motion in the particles of the body.. In the 
five methods enumerated by philofophers by which heat 
is produced, viz. by attrition, by chemical mixture, by 
fomentation, by inflammation, and by the fun, the heat 
depends on the tremulous or vibratory motion which by 
one means or another is excited in the particles of the 
heated body. We proceed to apply this theory of the 
generation of heat to the purpofe of proving that comets 
may be fit for the refidence of (material) intelligent 
creatures. 
The more plainly to illuftrate the manner in which 
this theory may be employed to fhow how comets can 
be habitable globes, it may be proper to apply it in ac¬ 
counting for a fad which comes under human obfervation. 
Within the torrid zone a perfon feels extreme heat at the 
foot of a high mountain, at the top of which the air is 
always in a freezing (late. At the foot of the mountain, 
the air, which is a very elaftic fubftance, being muchcom- 
prefied, i. e. the particles of it being brought very near to 
one another by the weight of the incumbent atmofphere, 
is there capable of receiving a greater degree of tremulous 
motion from the impulfe of the fun’s rays, and of thereby 
exciting the fenfation of great heat in a human body; 
whereas the air at the top of the mountain being not fo 
comprefled, but the particles of it, in confequence of its 
elafticity, and the fmall degree of prefigure from above to 
counteract that elafticity, being much farther removed 
from one another, is capable of receiving a proportionably 
lefs degree of tremulous motion from the impulfe of the 
fun’s rays : and therefore in that fituation the perfon 
feels no heat, but extreme cold, and in a fhort time 
would be frozen to death. In the latter fituation, how¬ 
ever, the perlon might be miles nearer to the fun than 
in the former. This fad then ferves to fliow that heat 
does not depend on the diltance alone of a body from 
the fun, but partly at lead, if not principally, on the 
quantity and depth of the elaftic medium on which its 
rays fall, and which they put into a vibratory motion : 
if we keep this obfervation in our minds, we (hall fee how 
this theory of the generation of heat may ferve to explain 
how comets may be no inconvenient places of refidence 
for fuch creatures as are before mentioned. Dr. Herfchel 
alfo afligns very forcible reafons for concluding that the 
opinion commonly received of the fun’s being a body 
of real fire is futile and erroneous. That the fun is a 
luminous and not an igneous body, has been thought 
highly probable by other modern philofophers, an opinion 
which every new ftep in philofophy gives great fupport to. 
The height of the atmofphere of a comet is frequently 
10,000 miles, whilft that of the atmofphere of the earth is 
hardly fuppofed to exceed 60 or 70. The particles of 
this atmofphere, much more than one hundred times as 
high as ours, may alfo be fmaller, more fubtle and elaftic, 
and much more eafily heated, than the particles of our 
atmofphere; whence the fun’s rays may be able to 
agitate and warm fuch an atmofphere comprefled 
together by the weight of 10,000 miles at a diltance 
from the fun, in which we, having over us an atmof¬ 
phere of only 60 or 70 miles, fliould feel the molt 
extreme cold. It is obfervable, that, as a comet moves 
towards the fun, it acquires a tail, which increafes its 
length the nearer the comet comes to the fun, and is 
longeft when the comet is nearelt to it. This tail we 
fuppofe to confifl of a quantity of the comet’s atmof¬ 
phere, driven behind it by the impelling force of the fun’s 
rays, which force, in regions near the fun, mull be great. 
In proportion as the tail, confifting of the atmofphere of 
the comet, lengthens behind it, the quantity of the 
fame atmofphere on the fide turned towards the fun 
grows iefs and lefs, and is leaft when the comet is nearelt 
599 
the fun. As, on the fide turned towards the fun, the 
quantity of atmofphere continually decreafes the nearer 
the comet approaches the fun, and again continually 
increafes the farther it recedes from it, there may be 
always fo much of it on that fide as is fufficient, by being 
put into a vibratory motion by the action of the fun’s 
rays upon it, for the purpofe of keeping up a proper 
degree of heat in the bodies of the inhabitants in every 
part of the orbit which the comet defcribes. Thus does 
this hypothelis concerning the manner in which heat is 
generated, enable us to conceive how a comet may be, 
like the earth, inhabited by rational beings, who may 
never fuffer any deftrudlive extreme of heat or cold, but 
enjoy, like us, a comfortable exiftence. 
PLAN'ET-STRUCK, udj. Dialled : 
Wonder not much if thus amaz’d I look ; 
Since I faw you I have been plctnet-flruck > 
A beauty, and fo rare, I did defcry. SucHiiig. 
PLANETA'RIUM, J. A machine for reprefenting tire 
motions of the planets by wdietlwork. This machine 
differs from an orrery in this refpedl, that it does not 
profefs to exhibit any of the diurnal rotations, but con¬ 
fines its operations to the production of the annual 
motions alone. Thefe motions may be either mean or 
equated ; the latter of which were firft exhibited, agree¬ 
ably to the Copernican fyftem, by Huygens’s auto¬ 
maton, in 1682; and the former by Roemer’s planetarium, 
foon after. The common planetarium, wdiicli prefents 
itfelf to the eye as we pafs the windows of the mathe¬ 
matical inftrument-makers, borrows the numbers of its 
planetary calculations chiefly from Huygens, and its con- 
itrudtion from the planetarium of Roemer, with little or 
no deviation, except what arifes front fuperior work- 
manfhip, and, in fome inftances, from the addition of the 
Georgian planet. The mean motions produced by this 
Ample mechanifm are far from being accurate ; and the 
errors, by continual accumulation, will become lenfible 
in a comparatively fhort fpace of time. But, where 
general reprefentation only is aimed at, and the refpedlive 
times of the phenomena exhibited by the planetarium are 
difregarded, this is the cheapeft and fimplelt conftrudlion 
of a planetary machine that has yet been deviled ; and, 
where its imperfedlions can be difpenfed with, its fim- 
plicity is no fmall recommendation. Still, however, the 
true places of the planets, depending on the equations as 
well as mean motions of thefe bodies, are not the places 
indicated by this planetarium, fuppofing its mean motions 
ever fo accurate ; nor are the motions of the five recently 
difcovered planets attempted to be exhibited, which they 
might be by additional wdieelwork, without altering the 
conftrudlion of the machine. When a planetarium of 
this common conftrudlion is fitted up, its appendages are 
ufually a lunarium and tellurium, feparately adapted to 
the fame ftand ; but in thofe, the annual and lunar trains 
are never free from confiderable errors. 
When the late Dr. Garnett refigned the ledlurefhip of 
the Royal Inftitution, and took his aftronomicai and 
other apparatus with him into Great Marlborough-ftreet, 
his fucceflor, Dr. Thomas Young, was authorifed by the 
managers of the inftitution to procure as good a plane¬ 
tarium for his ledlures as he could get for about fifty 
pounds ; this being confidered as large a fum as the ftate 
of the finances would allow to be expended on one inllru- 
ment, confidently with the other demands of the ap¬ 
paratus room. The dodlor, in confequence of the per- 
miflion thus given him, examined the fhops of the dif¬ 
ferent mathematical inftrument-makers for a machine 
already made, which might be confidered as adequate to 
the illuftration of the folar fyftem in a large and magni¬ 
ficent ledlure-room. His inquiries, however, were un- 
fuccefsful; but, having heard that aftronomicai mecha¬ 
nifm was a fubjedl which had occupied the attention of 
the Rev. W. Pearfon, he introduced himfelf to that gen¬ 
tleman in the autumn of 1801, for the purpofe of con¬ 
ferring 
