The enquirer. [volume] (Richmond, Va.) 1804-1815, 1811-10-29

are long and broad, and SHINE with stron-
ger light, appear more respicndent * more
exactly defined OF the convex than on the
concave side. Upon which accounts il is
pian that the /henomcna of the tails of CO
NETS depend upon the motions of their heads
and by no means upon the places of the hea-
vens i'm which their heads are seen & that,
therefore, the tails of comets do not proceed
from the retr;;ction o. the heavens, but from
their own heads, which furnish the matter
that forms the tail. For as in our air, the
smoke OF heated body ascends either per
pendicnlarly if the body is at rest or oblit,ne-
~ II the body is moved obliquely, SO in the
heavens, where all bodies gravitate towards
the sun, smoke & vapour must Gas we have
already said) ascend from the sun, & either
rise perpendicularly if the smoking body is
at rest, or o5iqueiy if the body, in all the
progress OF its motion, is always LEAVING.
those places frop which the upper or high
er parts of the vapour had risen before *
that obliquity viii be least where the vapor
ascends with most velocity, to wit, near the
smoking body, when that is near the SUM-
hut,because the obliquity varies, the column
OF vapour will be incurvated and because
the vapour in the preceding sides is some
thing more recent, that is, has ascended
something more late from the body, it will
therefore be something more dense on that
sioe, and must on that account reflect more
light, as well as be better defined. I add
nothing concerning the sudden uncertain a.
CITATION OF the tails OF comets, and their ir-
regular figures, which authors sometimes
describe, because they may arise from the
mutations of orr ai,, and the motions of o,r
clouds, IN Part cbscufing these tails or,
perhaps, from parts of the Via Lactca.
which might have been confounded with &
mistaken Tor parts of the tails of the comets
as they passed by
But that the atmospheres of comets NAV
furnish supply of vapour great enough to
bii 50 immense spaces,we NAV easily under
stand from the rarity of our own air for
The air near the surface of our earth posses
ses space $50 times greater than water of
u.e same weight and therefore a cylinder
u ar CSC feet high is of equal weight with
q cylinder of water of the same breadth. &
out one foot high. But a cylinder of air
reaching to the top of the atmosphere is of
elual weight with cylinder of water about
% feet high: and, therefore, if from the
whole CYLINDER of air the lower part of $50
-cet h'Sh is taken away, the remaining up
per Part VIII be OF equal weight with cy-
Under q water 80 feet high and from
thence (and by the hypothesis, confirmed by
many experiments, that the compression of
arr is as the weight of the incumbent atmos-
phere, and that the force of gravity is reci-
Procaily as the square Of the distance from
TIC centre OF the earth) raising calculus.
by Cor. prop. %, book 2, found that, at the
he'ght OF one semi-diameter of the earth,
reckoned from the earth's surface, the air is
more rare than with H. in far greater pro
portion than OF the whole space within the
Old Oh saturn to spherical space of one
lnch In diameter (; and therefore if a sphere
of our air of but one inch in thickness was g.
iually rareticd with the air at the height of
one seml-diameter of the earth from the
eirths surface, 1t would fill all the reqions
OF the planets to the orb of Saturn, and far
beyond 1t. Wherefore since the air at e1ea-
ter distances is immensely rarefed. and the
coma or atmosphere of comets is ordinarily
ABOUT ten times higher, reckoning from their
centres tnah the surface of the nucleus, and
the tails rise yet higher, they must there
fore be exceedingly rare and though On
account OF the much thicker atmospneres
OF comets, and the great gravtation of their
oodles towards the sun, as well as of the
particles OF their air and vapours mutually
one towards another, it may happen that the
ar IN the celestial spaces and in the tails
OF comets is not so vastly rarefied, vet from
thls computation it is plain that a very small
Quantity OF air and vapour is abunilantly suf-
hclent to produce all the appearance of the
taiis OF comets Tor that they are, indeed,
OF a very notable rarity appears from the sbi-
ming OF the stars through them. The at
mosphere OF the earth, Illuminated by the
suns light, though but of few miles in
thickness, quite closures and ext;nguishes
the light not only of all the stars, but even
OF the moon itself whereas the smallest
stars are seen to shine through the immense
thickness OF the tails of comets. likewise IL
Illuminated by the sun, without the least di
mlnution OF their splendor Nor is the
brightness OF the tails of most comets ord;
narily greater than that of our air, an inch
or two III thickness, rejecting in darkened
room the light of the sun-bcams let in by a
hole OF the window shutter.
And we may pretty nearly determine
the time spent during the ascent of the va
pour from the comets head to the extremity
OF the tan, by drawing right line from the
extremity OF the tail to the sun, and mark
ing the place where that right line7inter-
Sects the Comets orbit for the vaDour TIN at
is now III the extremity of the tail, if il has
assembled III right line from the sun must
have begun to rise from tle head at the time
when the head was in the point of iutersct
ton. lie Is true, the vapour does not rise in
right line from the sun but, RETAINING 4he
motion which it had from the conc, before ;ts
ascent, < ccnpouad.ng that motion with its
motion OF asceat,ailscs 0bHJuey; & therefore
the solution Of the problem viii be more e,,
act, II we draw the iiuc which intcrsect, the
orbit parallel to the length of the tail or
rather (hecan2e Of The ctrviiinear motion of
the comct) dNcrSing a little from thc li.;c
or ie.,gtN o; thc tail. And by means Of tn:s