ATM

M/5

ATR

r I hen, *o determine the value of the general quantity b, let us takea case m which we know the altitude a, which corresponds to aknown density d ; as for instance, taking a — 1 foot, or 1 inch, orsome such small altitude; then, because the density D may bemeasured by the pressure of the whole atmosphere, or the uniformcolumn of 27600 feet, when, the temperature is 50°; therefore27600 feet will denote the density D at the lower place, and27599 the Jess density d at 1 foot above it ; consequently 1 — b27600

X log. oi — , which by the nature of logarithms, is nearly

.4342944S

? ■ —nearly; and hence we find b -Ooo51

— bx 27600

63551 feet; which gives us this formula for any altitude a in ge-

D M

neral, viz. a = 63551 x log of-r-, or«=6355l xlog of—feet, o>-

d m '

10592 X log. of —fathoms ; where Mdenotes the column of mer-m

cury in the tube at the lower place, and m that at the upper. Thisformula is adapted to the mean temperature of the air 55” : but ithas been found, by the experiments of Sir George Shukhurgh andGeneral lloy, that for every degree of the thermometer, differentfrom 59°, tire altitude a will vary by its 435th part; hence, if wecould change the factor b from 10592 to 10000, because tiie diffe-rence 892 is the 18th part of the whole factor 10592, and because18 is the 24th part of 435 ; therefore the change of temperature,answering to the change of the factor b is 24° ; which reduces the

M

55° to 31°. So that, u = 10000 x log. of — fathoms, is the easiest

expression for the altitude, and answers to the temperature Qf 31°,or very nearly the freezing point; and for every degree abovethat, tne result must be increased by so many times its 435th part,and diminished when below it.

From this theorem it follows, that, at the height of 3-J miles, thedensity of the atmosphere is nearly twice rarer than it is at the sur-face of the earth ; at the height of 7 miles, 4 times rarer; and soon, according to the following table:

Number of times rarer.

H

O

7

4

14

16

21

64

28

256

35

1024

42

4096

49

16384

56

65536

63

262144

70

1048576 1

And, by pursuing the calculations in this table, it might be ea-sily'shewn, that a cubic inch of the air we breathe would be somuch rarefied at the height ol 500 miles, that it would till a sphereequal in diameter to the orbit of Saturn . Hence if all the pla-nets have atmospheres, they will be so attenuated at the dis-tances of the planets front one another, as to give no sensible re-sistance to their motions round the sun, for thousands of ages.Hence, though the atmosphere extends indefinitely, yet at smallaltitudes it is so rare as not to affect the particles of light whichare the smallest we are acquainted with, and at this altitude theboundary of the atmosphere has been fixed, and has on calculationbeen found to be about 40 or 50 miles, as appears from live du-ration of the twilight, and from the magnitude of the terrestrial sha-dow in lunar eclipses-.

Light is refracted by the atmosphere, causing thereby an appa-rent elevation of the heavenly bodies. See Astronomy (Index).This refraction is also the cause of several other pluenomena. Theatmosphere, or air, has also a reflective power; and this power isthe means by which objects are enlightened so uniformly on allsides. The want of this power would occasion a strange alterationin the appearance of things ; the shadows of which would b<- sovery dark, and their sides enlightened by the sun so very bright,that probably we could see no more of them than their bri'ditparts; so that for a view of the other parts, we must turn themhalf round, or if immoveable, must wait till the sun could comeround-upon them. Such a pellucid unrefleclive atmosphere would

VOL. I.—NO. 22.

indeed have been very commodious for astronomical observationson the course of the sun and planets among the iixed stars, visibleby day as well as by night; lmtthen such a sudden transition frontdarkness to light, and from light to darkness, immediately uponthe rising at.d setting of the sun, without any twilight, and evenupon turning to or fre m the sun at noon-day, would have beenvery incovcnient and offensive to our eyes.

Atmosphere, Salubrity op the. Although it is agreeableto experience, that dense air is more proper for respiration, titanair that is extremely rant: yet the air of mountains, by reason ofits purity, is found to be more salubrious than the air of pits andlow situations, where the atmospheric air is generally tainted In-noxious vapours. M. de Saussure concludes, that the air betweenthe two extremes, or about the height of 500 or 600 yards above-the level of the sea, is the most salubrious and proper for animallife. Other causes, besides the mere difference ot altitude, atfvetthe salubrity of the atmosphere. Air when confined and stagnantis less pure than when it is agitated and shifted, hence, the air in abcd-chamber is less salubrious in a morning, alter it has been sleptin, than in an evening. The air in some cases has been found re-markably impure after close and sultry weather, and Dr. Ingen-house remarks, that the air at sea, and close to it, is in generalpurer anil fitter for animal life than the air on land, but thereseems to be little difference between the air of towns, and that ofthe country> or between the air of one town and that of another.Mr. Fontana has made the same conclusions, and farther adds, thatleaving out of tlie account some particular places, the air is almostequally- good every where, and at every time, and that small dif-ferences are not so much to he feared, as some people would haveus to believe.

Atmosphere, in electricity-, is the. air or fluid which surroundselectrified bodies, aud is affected by the electricity.

ATOCK, a province in tire dominions of the Great Mogul;also the capital of the province. Long. 72. It). E. Lat. 32,20. N.

ATOMICAL Philosophy, or tire doctrine- of atoms, asystem which, from the hypothesis that atoms are endued withgravity and motion, accounted for the origin and formation ofthings. Tliis philosophy Wits first broached by- Moschus, sometime before the Trojan war; but was much cultivated and improvedby" Epicurus ; whence it is denominated the Epicurean philosophy.

ATOMS, [etTo/z©-, Gr.] such small particles as cannot he physi-cally divided : and these are the first rudiments, or the componentparts of all bodies.

Atonement. See Expiation.

ATONTCS, in grammar, words unaccented.

ATONY, [from «, and tot-, tone,] in medicine, a defect oftone or tension, or a laxity or debility of the solids of the body.

ATOOl, one of the Sandwich islands, about 300 miles in cir-cumference. Long. 200. 20. E. latt. 21. 57. N.

ATHAWLIS, BLACK BILE , or M ELANCFIOLY, ac-cording to the ancients, it hath a twofold origin: 1st, From thegrosser parts of the blood, and this they call the “ Melancholy Hu-mour.” 2d, From yellow bile being highly concocted. Dr. lYr-cival, suggests, that it is the gall, rendered acrid, by a stagnationin the gall-bladder, and rendered viscid, by the absorption of allits fluid parts. Bile in this state discharged into the duodenum,occasions universal disturbance and disorder, until it is evacuatedit occasions violent vomiting, purging, head-ach, delirium, fire.

ATUACTYLIS, DistaffThistlk: A genus of the polvgamiaicqualis order, and syngenesia class of plants; and in the naturalorder, Composite eapilatre, with radiated flowers and ci repressedseeds, coronated with a plumose down, and standing on a planevillose receptacle. There are eight species.

ATR A Dies, in antiquity, denotes a fatal day whereon the Ho-mans received some memoiahle defeat. The word literally- im-ports a black day ; a denomination taken from the colour, whichis Ihe emblem ot death and mourning. The dies alrtc, or atri.were denominated nefasli, and posteri.

ATR AGENE, in botany, a genus of the polyandria order, andpolygynia class of plants. Calyx four-leaved; petals twelve ; seedscaudated, There are live species, ail natives of the east.

ATRAl’lIAKlS, a genus of the digynia order, and hexandriaclass of plants ; aud in the natural order, Iloloracra*. Calyx two-leaved; petal* two, sinuated ; seed one. There are two species,both natives of warm countries.

4 T ATREBATES