A T JVT 
that this'diameter increafes, till at lad, when the rays are 
perpendicular, it becomes infinite, or the circle degenerates 
into a right line. This reafoning fuppofes that the retract¬ 
ing furface of the atmofphere is a plane ; but fince it is 
in reality a curve, he obferves that thefe tycloids become 
in fact epicycloids. But Herman detedted the error of 
M. de la Hire, and (hewed that this curve is infinitely ex- 
tended, and has an afymptote. And it isobferved by Dr. 
Brook Taylor, in his Methodus Increm. p.168, &c. that 
this curve is one of the mod intricate and perplexed that 
can well be propofed. The fame ingenious author com¬ 
putes, that the refractive power of the air is to the force 
of gravity at the furface of the earth, as 320,000,000 to 1. 
Confidering the extreme rarity of the atmofphere at on¬ 
ly forty or fifty miles in height, it feems to be furpriziug 
that fome meteors fhould be inflamed at fuch great heights 
as they have been feen. A very remarkable one of this 
kind was obferved by Dr. Halley in the month of March 
1719, the altitude of which he computed at between fix- 
ty-nine and feventy-three and an half Englifh miles ; its 
diameter 2800 yards, or more than a mile and an half; and 
its velocity about 350 miles in a minute. Others, appa¬ 
rently of the fame kind, but whofe altitude and velocity 
were ftill greater, have been obferved ; particularly that 
very remarkable one, of Auguft 18, 1783, whofe difiance 
from the earth could not be lefs than ninety miles, its di¬ 
ameter at leaft as large as the former, while its velocity was 
certainly not lefs than 1000 miles in a minute. Now, from 
analogy of reafoning, it feems very probable, that the me¬ 
teors which appear at fuch great heights in the air, are not 
effentially different from thofe which are feen on or near 
the furface of the earth. The difficulty with regard to 
the former is, that, at the great heights above-mentioned, 
the atmofphere ought not to have any denfity fufficient to 
fupport flame, or to propagate found ; and yet fuch me¬ 
teors are commonly fucceeded by one explofion or more, 
and it is faid are even fometimes accompanied with a hif- 
fing noife as they pafs over our heads. The meteor of 
1719 was not only very bright, feeming for a fliort time 
to turn night into day, but was attended with an explo- 
fion heard over all the ifland of Britain, caufing a violent 
concuffion in the atmofphere, and feeming to (hake the 
earth itfelf. And yet, in the regions in which this me¬ 
teor moved, the air ought to have been 300,000 times 
rarer than that of the air we breathe, or 1000 times rarer 
than the vacuum commonly made by a good air-pump. 
Dr. Halley offers a conjedture, indeed, that the vaft mag¬ 
nitude of fuch bodies might compenfate for the thinnefs 
of the medium in which they moved. But appearances 
of this kind are, by fome others, attributed to electricity ; 
though the circumftances of them cannot be reconciled to 
that caufe; for the meteors move with all different degrees 
of velocity; and, though the eledtrical fire eafily pervades 
the vacuum of an air-pump, yet it doe* not in that cafe 
appear in bright well-defined (parks, as in the open air, 
but rather in long dreams refembling the aurora borealis ; 
and from fome late experiments it has been concluded that 
the eledtric fluid cannot even penetrate a perfedt vacuum. 
It has been obferved above, that the atmofphere has a 
refradtive power, by which the rays of light are bent from 
the right-lined diredtion, as in the cafe of twilight; and 
many other experiments manrfeft the fame virtue, which 
is the caufe of many phenomena. Alhazen, the Arabian, 
who lived about the year 1100, was more inquifitive into 
the nature of refraction than former writers. But neither 
Alhazen, nor his follower Vitella, knew any thing of its juft 
quantity, which was not known, to any tolerable degree 
of exadtnefs, till Tycho Brahe, with great diligence, fet¬ 
tled it. But neither did Tycho nor Kepler difcover in 
what manner the rays of light were refradted by the at¬ 
mofphere. Tycho thought the refraction was chiefly 
caufed by denfe vapours, very near the earth’s furface : 
while Kepler placed the caufe wholly at the top of the at¬ 
mofphere, which he thought was uniformly deufe ; and 
Vo l. II. No. 84. 
A T O 481 
thence he determined its altitude to be little more titan 
that of the higheft mountains. But the true conftitution 
of the denliiy of the atmofphere, deduced afterwards 
from the Torricellian .experiment, afforded ajufter idea' 
of thefe refradtions, efpecially after it appeared, by a re¬ 
petition of Mr. Lowthord’s experiment, that the retrac¬ 
tive power of the air is proportional to its denfity. By 
this variation in the denfity and refractive power of the 
air, a ray of light, in palling through the atmofphere, is 
continually refradted at every point, and thereby made to 
defcribe a curve, and not a ftraight line, as it would have 
done were there no atmofphere, or were its denfity uni¬ 
form ; as is (hewn under Astronomy. 
The atmofphere has alio a refledtive power; and this 
power is the means by which objedts are enlightened fo 
uniformly on all Tides. The want of this power would 
occafion a ftrange alteration in the appearance of things ; 
the ffiadows of which would be fo very dark, and their 
fides enlightened by the fun fo very bright, that probably 
we could fee no more of them than their bright halves; 
fo that for a view of the other halves, we muft turn them 
half round, or, if immoveable, muft wait till the fun could 
come round upon them. Such a pellucid unrefledtive at¬ 
mofphere would indeed have been very commodious for 
aftronomical obfervations on the courfe of the fun and pla¬ 
nets among the fixed ftars, vifible by day as well as by 
night ; but then fuch a fudden tranlition from darkaefs to 
light, and from light to darknefs, immediately upon the 
riling and fetting of the fun, without any twilight, and 
even upon turning to or from the fun at noon-day, would 
have been , very inconvenient and injurious to our eyes. 
However, though the atmofphere be greatly alliftant ifi 
the illumination of objedts, yet it muft alfo be obferved 
that it flops a great deal of light. By M. Bouguer’s ex¬ 
periments, it feems that the light of the moon is often 
2000 times weaker in the horizon than at the altitude of 
fixty-fix degrees ; and that the proportion of her light at 
the altitudes of fixty-fix and nineteen degrees, is about 3 
to 2; and the lights of the fun muft bear the fame pro¬ 
portion to each other at thofe heights ; which Bouguer 
made choice of, as being the meridian heights of the fun, 
at the fummer and winter folftices, in the latitude of Croific 
in France; effects that are folely produced by the matter 
or fubftance of the atmofphere. 
ATMOSPHE'RICAL, adj. Confiding of the atmo¬ 
fphere ; belonging to the atmofphere.—We did not men¬ 
tion the weight of the incumbent atmofpherical cylinder, as 
a part of the weight refilled. Boyle. 
AT'MUS,yi [ciTf/.oj, from to breathe.] Vapour, 
breath, flatus. 
ATO'CIA,^ [«Toxia, from a neg. and tixtco, to bring 
forth.] Inability to bring forth children. Difficult labour, 
ATO'CUS,/ [a tcixo;, from a. neg. and rixTo, to bring 
forth.] Barren; not able to procreate. 
ATOL'LI,y. A fort of pap, made of the meal of maize 
and water, which the Indians mix with their chocolate. 
ATOL'MI A,yi [arci/y.ia, from « neg. and to^ccu, to 
dare.] Diffidence, dejedtion of mind. 
ATOM, f. [atome , Fr. atomus, Lat. of are/*©-, of at 
priv. and Tty.uo, Gr. to cut or divide.] Such a fmall par¬ 
ticle as cannot be phyfically divided : and thefe are the 
firft rudiments, or the component parts, of all bodies.— 
Innumerable minute bodies are called atoms , becaufe, by 
reafon of their perfedt folidity, they were really indiviu- 
ble. Ray. 
See plaftic nature working to this end, 
The Angle atoms each to other tend, 
Attradt, attradted to, the next in place 
Form’d and impell’d its neighbour to embrace. Pope. 
Any thing extremely fmall.—It is as eafy to count atoms a 
as to refolve the propofitions of a lover. Shakefpeare. 
ATO'MICAL, adj. Confiding of atoms.—Vitrified and 
pellucid bodies are clearer in their continuities, than in 
6 G powders 
