Encyclopaedia Britannica > Volume 7, ETM-GOA

FIR [ 249 ‘] FIR
which laft they fometimes refemble in being attended
with a train ; but frequently they appear with a round
and well defined dilk. The firft of thefe of which we
have any accurate account, was obferved by Dr Halley
and fome other philofophers at different places, in the
year 1719. From the flight obfervations they could
take of its courfe among the ftars, the perpendicular
height of this body was computed at about 70 miles
from the furface of the earth. The height of others
has alfo been computed, and found to be various;
though in general it is fuppofed to be beyond the li¬
mits affigned to our atmofphere, or where it lofes its
refradfive power. The moil remarkable of thefe on
record appeared on the 18th of Auguft 1783, about
nine o’clock in the evening. It was feen to the north¬
ward of Shetland, and took a foutherly diredtion for
an immenfe fpace, being obferved as far as the fouthern
provinces of France, and one account fays that it was
ieen at Rome alfo. Duringitscourfe it appears frequently
to have changed its fliape; fometimes appearing in the
form of one ball, fometimes of two or more ; fometimes
with a train, fometimes without one. It paffed ov.er E-
dinburgh nearly in the zenith, and had then the appear¬
ance of a well defined round body, extremely luminous,
and of a greenifh colour ; the light which it diffufed on
the,ground giving likewife a greenifh caftto objefts. Af¬
ter pa fling the zenith it was attended by a train of confi-
derable length, which continually augmenting, at laft
obliterated the head entirely ; fo that it looked like a
wedge, flying with the obtufe end foremoft. The
motion was not apparently fwift, by reafon of its great
height; though in reality it mull have moved with
great rapidity, on account of the vaft fpace it travelled
over in a fhort time. In other places its appearance
was very different. At Greenwich we are told, that
“ two bright balls parallel to each other led the way,
the diameter of which appeared to be about two feet;
and were followed by an expulfion of eight others, not
elliptical, feeming gradually to mutilate, for the laft
was fmall. Between each two balls a luminous ferra-
ted body extended, and at the laft a blaze iflued which
terminated in a point. Minute particles dilated from
the whole. The balls were tinted firft by a pure
bright light, then followed a tender yellow, mixed
with azure, red, green, &c. ; which, with a coalition
®f bolder tints, and a refledtion from the other balls,
gave the moft beautiful rotundity and variation of co¬
lours that the human eye could be charmed with. The
fudden illumination of the atmofphere, and the form
and Angular tranfition of this bright luminary, tended
much to make it awful: neverthelefs the amazing vi¬
vid appearance of the different balls, and other rich
conne&ive parts not veryeafy to delineate, gave an effedf
equal to the rainbow in the full zenith of its glory.”
Dr Blagden, in a paper on this fubjeft in the 74th
volume of the Philofophical Tranfaftions, has not on¬
ly given a particular account of this and other meteors
of the kind, but added feveral conje&ures relating to
the probable caufes of them. The firft thing which
occurred to philofophers on this fubjedt was, that the
meteors in queftion were burning bodies rifing from
the furface of the earth, and flying along the atmo¬
fphere with great rapidity. But this hypothefis was
foon abandoned, on confidering that there was no
power known bv which fuch bodies could either be
Von. VII. Part I.
raifed to a fufficient height, or projedted with the ve¬
locity of the meteors. The next hypothefis was, that
they do not confift of one Angle body, but of a train of
fulphureous vapours, extending a vaft way through the
atmofphere, and being kindled at one end, difplay the
luminous appearances in queftion by the fire running
from one end of the train to the other. To this hypo-
thefis, which was invented by Dr Halley, Dr Blagden
objedfs that no juft explanation is given of the natura
of the vapours themfelves, the manner in which they are
raifed up, or in which they can be regularly arranged in
ftraight lines of fuch vaft extent; or how they can be
fuppofed to burn in fuch rarefied air. “ Indeed,
(fays he) it is very difficult to conceive how vapours
could be prevented, in thofe regions where there is in a
manner no preffure, from fpreading out on all Aides in
confequence of their natural elafticity, and inftantly
lofing that degree of denfity which feems neceffary for
inflammation. Befides, it is to be expedted, that fuch
trains would fometimes take fire in the middle, and
thus prefent the phenomenon of two meteors at the
fame time, receding from one another in a diredf line.”
For thefe and ether reafons this hypothefis of Dr
Halley was abandoned, and another fubftituted in its
place. This was, that the meteors we fpeak of are
permanent fdlid bodies, not rifing from the earth, but
revolving round it in very eccentric orbits, and thus
in their perigeon moving with inconceivable rapidi¬
ty. But the Dodfor fhows, that even on this fup-
pofition, the velocity of fuch bodies muft fcarce be
one third of that with which fire-balls move, and
which has been calculated at upwards of 1000 miles
per minute. The hypothefis is likewife liable to a
number of other objedfions -which cannot be anfwer-
ed, particularly from the variations in their appear¬
ance ; for it is impoffible to fhow in what manner one
folid and permanent body could affume the appear¬
ance of eight or ten, as was the cafe with the meteor
of 1783 ; nor can it be ftiown why a body, which in
palling over Edinburgh appeared with a*dilk evident¬
ly lefs than that of the fun, Ihould, in palling over
Greenwich, affume the appearance of two bodies, each
of which had a dilk coniiderably larger than the ap¬
parent dilk of that luminary. To obviate, in forne
meafure, objedlions of this kind, it has been fuppofed
that the revolving bodies are furrounded by a kind of
eledtrical atmofphere by which they are rendered lu¬
minous ; “ but (fays the Dodlor) I think, whoever
carefully perufes the various accounts of fire-balls, and
efpecially ours of the 18th of Auguft, when it divided,
will perceive that their phenomena do not correfpond
with the idea of a folid nucleus involved in a fubtle
fluid, any more than with the idea of another learned
gentleman, that they become luminous by means of a
contained fluid, which occafionally explodes through
the thick folid outer Ihell.”
Another hypothefis, which DrBlagden has not men¬
tioned, is, thatthemeteorsinqueftion areakind of bodies
which take fire as foon as they come within the atmo¬
fphere of the earth. But this cannot be fuppofed with¬
out implying a previous knowledge of thefe bodies,
which it is altogether impoffible we can have. The
only opportunity we have of feeing them is when they
are on tire. Before that time they are in an invilible
and unknown ftate ; and it is furely improper to ar-
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