HISTORY OF PHYSICAL ASTRONOMY.

225

have an intimate connexion with the rotation of the sun about his axis.The remarkable fact of their absence from the polar regions, and theirconfinement to two zones, extending to about 35° on each side of theequator, with an intermediate equatorial belt in which they are morerarely perceived, appears to him to afford an indubitable indication thatthey derive their existence from circulating movements in the atmosphericfluid which encompasses the sun, depending on the rotation of that bodyabout its axis, by a relation similar to that which connects the system ofterrestrial trade and anti-trade winds with the earth’s rotation. Now ifany cause can be assigned which is capable of creating a circulation of theatmospheric fluid between the equator and the poles, the effect of thesun’s rotation will be to modify the currents so produced—as the tradewinds and monsoons on the surface of the earth are modified—and to dis-pose all the meteorological phenomena which accompany them as theirvisible manifestations into two zones parallel to the equator, with a calmequatorial zone interposed. The question then arises, where are we todiscover the exciting cause of such circulations in the fluids of the solaratmosphere ? With reference to this point, Sir J. Herschel remarksthat the trade winds are occasioned by the unequal effects of thesolar rays in heating the equatorial and polar regions of the ter-restrial atmosphere; but, as no such external cause exists in thepresent case, we are necessarily led to look for the source of circulation inthe economy of the sun itself. He then proceeds to state that if there canbe assigned any physical difference in the constitutions of the equatorialand polar regions of the sun, which tends to repress the escape of heat inthe one of these regions, and to favour it in the other, the effect will be thesame as if these regions were unequally heated and all the phenomena oftrade winds mutatis mutandis must arise. Before proceeding to a furtherexposition of his views, he takes this occasion to remark that the exist-ence of a solar atmosphere cannot admit of any doubt. He adducesin support of his assertion the deficiency of light exhibited by the borderof the solar disk, whether when viewed through coloured glasses, or byprojecting its image on white paper. But a phenomenon which appearsto him to establish still more conclusively the existence of such an atmo-sphere, is that which was witnessed during the total eclipse of the sun whichoccurred on the 8th of July, 1842*. On that occasion there appearedthree rose-coloured protuberances of immense size projecting from thedark limb of the moon, which could not have been any other than cloudsfloating in, and sustained by, a transparent atmosphere enveloping thesun. To what distance from the surface of that body this atmospheremay extend, it is difficult to ascertain with precision; but, from the de-ficiency of light exhibited by the exterior part of the disk being gradual,and also perceptible at some distance within the margin, he is induced tosuppose that it must be considerable, not merely in absolute measure, butas an aliquot part of the sun's radius. Admitting, then, the existence ofsuch an atmosphere, the laws of equilibrium will cause it to assume thefigure of an oblate spheroid, and the gaseous envelope so formed beingthicker round the equator than at the poles, a different obstacle will beopposed to the escape of heat from the equatorial and the polar regions ofthe sun. The manifest result will be, that an habitual difference of tem-perature will prevail in the two regions.

* For interesting accounts of this eclipse, drawn up from personal observation, seepapers by Baily and Airy, in vol. xv. of the “ Memoirs of the Astronomical Society.”

Q