Z O D 



M. Fatio conjeftured that this appearance arifes from a 

 colleftion of corpufcles encompaffing the fun in the form 

 of a lens, refledling the light of the fun. M. Caflini fup- 

 pofed that it might arife from an infinite number of 

 planets revolving about the fun ; fo that this light might 

 owe its exiftence to thefe bodies, as the milky way does to 

 an innumerable number of fixed ftars. It is now, however, 

 generally fuppofed, that it is matter detached from the fun 

 by its rotation about its axis. The velocity of the equato- 

 rial parts of the fun being the greateft would throw the 

 matter to the greateft diftance, and on account of the dimi- 

 nution of velocity towards its poles, the height to which 

 the matter would there rife would be diminifhed ; and as it 

 would probably fpread a little fideways, it would form an 

 atmofphere about the fun fomething in the form of a lens, 

 whofe feftion perpendicular to its axis would coincide with 

 the fun's equator. And this agrees very well with obfer- 

 vatioH. There is, however, a diiBculty in thus accounting 

 for this phenomenon. It is very well known that the cen- 

 trifugal force of a point of the fun's equator is a great many 

 times lefs than its gravity. It does not appear, therefore, 

 how the fun, from its rotation, can detach any of its grofs 

 particles. If they be particles detached from the fun, they 

 muft be fent off by fome other unknown force ; and in that 

 cafe they might be fent off equally in all diredtions, which 

 would not agree with the obferved figure. The caufe is 

 probably owing to the fun's rotation, although not imme- 

 diately to the centrifugal force arifing therefrom. 



The zodiacal light, according to M. de Mairan's inge- 

 nious and plaufible hypothefis, is nothing but the folar 

 atmofphere, a rare and fubtile fluid, either luminous by 

 itfelf, or made fo by the rays of the fun furrounding its 

 globe ; but in a greater quantity, and more extenfively 

 about its equator than any other part. As it always 

 accompanies the fun, it is natural to afcribe it to a folar 

 atmofphere, extending beyond the orbit of Mercury, and 

 fometimes even beyond that of Venus. Accordingly, the 

 zodiacal light has been fuppofed to be a feftion of this 

 atmofphere, which, being extremely flat at its poles, cannot 

 be conceived to partake of the fun's monthly motion. Dr. 

 T. Young (Ledlures, vol. i. p. 502.) obferves, that the 

 only probable manner in which it can be fuppofed to retain 

 its figure, is by means of a revolution much more rapid 

 than that of the fun's motion. To that purpofe, M. de la 

 Lande remarks, that it feems now to be generally believed, 

 that the zodiacal light is the atmofphere of the fun ; for it 

 always accompanies that luminary ; and the equator of the 

 fun is in the direftion of this light : confequently he fays, 

 that in all probability the zodiacal light is an atmofphere 

 fituated round the fun, in the direflion of its equator, and 

 flattened by its rotatory motion. Aftrom. Paris, 1771. 

 § 845 to 849. 



The zodiacal light is more or lefs vifible according to 

 circumftances ; but the folar atmofphere is not always 

 vifible by means of this fight, though it be always feen 

 about the globe of the fun in total echpfes. 



One of the moft effential circumftances for the percep- 

 tion of the folar atmofphere by the zodiacal light is its 

 having fufficient length on the zodiac ; for without this 

 its brightnefs is entirely hid from us by the twilight. 



M. de Mairan fays, it may be proved from many obfer- 

 vations, that the fun's atmofphere fometimes reaches as far 

 as the earth's orbit, and there meeting with our atmofphere 

 produces the appearance of an aurora boreahs. 



The length of the zodiacal light varies fometimes in rea- 

 lity, and fometimes in appearance only, from various caufes. 



The oblique pofition of this light, little diiferent from 

 Vol. XXXIX. 



Z O E 



■''^'•^^n? P'^"^ °^^^^ ecliptic, does not permit us to fee 

 It diitinftly, and fufSciently elevated above the horizon • 

 but fome time after fun-fet, towards the end of the winterj 

 and m fpnng, or before fun-rifing in autumn, and towards 

 the begmning of winter. Several caufes hinder our feeing 

 It, any more than the milky way ; fuch as moon-light 

 and Itrong twilights, among others. 



M. Caflini often mentions the great refemblance of the 

 zodiacal light to the tails of comets. M. Fatio has made the 

 lame obfervation ; and M. Euler has lately endeavoured to 

 prove them owing to fimilar caufes. Decouverte de la Lu- 

 ™\'!f C'^^^fte que paroit dans le Zodiaque, art. 41. Lettre 

 a M. Caflini, printed at Amfterdam, 1686. Euler, in 

 Mem. de I'Acad. de Berlin, torn. ii. 



The figure of this folar atmofphere muft be lenticular, 

 or that of a flatted fpheroid. M. de Mairan gives us a 

 draught of its appearance and projeftion. 



The extent of the zodiacal light from the fun to its point 

 is feldom lefs than 45, fometimes 150 degrees in length ; M. 

 Pingre being on the torrid zone faw it 1 20 degrees : and its 

 breadth varies from 8 to 30 degrees. 



This light feems to have no other motion than that of the 

 fun itfelf. 



M. Euler obferves, that if the fun has an atmofphere, 

 the force of the impulfe of light iffuing from that globe 

 muft drive particles of that atmofphere before it ; but as 

 gravity is very ftrong at the fun, this impulfe would never 

 drive thofe particles beyond the limits of their atmofphere, 

 were it not for the centrifugal force arifing from the fun's 

 motion round its axis. This being oppofite to the aftion of 

 gravity, diminifliing its effefts, the impulfe of the light may 

 confiderably dilate the figure of the folar atmofphere, from 

 what it would be if it arofe from the gravity and centrifugal 

 force of its particles only : and this dilatation will be very 

 confiderable near the fun's equator, and very fmall towards 

 its poles. The aftion of light thus diminifliing the aftion of 

 gravity, M. Euler attempts to calculate how far this dimi- 

 nution of gravity may increafe the extent of the fun's atmo- 

 fphere about its equator. He finds a cubic equation, the 

 roots of which exprefs the femi-axis, or greateft amplitude 

 of this atmofphere. He adds, that this equation having 

 three real roots, it is pofltble that tlie folar atmofphere 

 may become a ring furrounding the fun's globe, as the ring 

 of Saturn furrounds the body of that planet. As the 

 eleftric fluid is now generally acknowledged to be the caufe 

 of the aurora borealis, which M. de Mairan afcribes to the 

 folar atmofphere, which produces tlie zodiacal light, and 

 which is thrown off principally and to the greateft diftance 

 from the equatorial parts of the fun, in confequence of his 

 rotation on his axis, and extending vifibly, in the form 

 of a luminous pyramid, as far as the orbit of the earth, falls 

 into the upper regions of our atmofphere, and is coUefted 

 chiefly towards the polar parts of the earth, in confequence 

 of the diurnal revolution, where it forms the aurora boreahs : 

 it is no improbable conjefture, that the fun may be the 

 fountain of the elettric fluid, and that the zodiacal light, 

 and the tails of comets, as well as the aurora borealis, light- 

 ning, and artificial electricity, are its various and not very 

 diflimilar modifications. See Theory 0/ Electricity. 



ZODZISZKI, in Geography, a town of Lithuania, in 

 the palatinate of Wilna ; ^l miles E. of Wilna. 



ZOEGEA, in Botany, was fo named by Linnaeus, in 

 honour of his pupil and correfpondent Dr. John Zoega, 

 who vifited Iceland, and communicated from thence, as well 

 as from the neighbouring ocean, many new or rare plants, 

 efpecially of the cryptogamic tribes, to the great Swedifli 

 naturalift. Dr. Zoega wrote a Flora Idandka, which has 

 C c been 



