at A, and would be seen projected on C, the centre of the sun. It is actually at Q, projected upon D, at an apparent distance CD to the north of the centre, which is the apparent smaller semi-axis of the ellipse described by the spot, which being C D known by micrometric measurement, the value of

or the

CN cosine of QC N, the inclination of the sun's equator becomes known, CN being the apparent semi-diameter of the sun at the time. At this epoch, moreover, the northern half of the circle described by the spot is visible (the southern passing behind the body of the sun), and the south pole p of the sun is within the visible hemisphere. This is the case in the whole interval from December 11th to July 12th, during which, the visual ray falls upon the southern side of the sun's equator. The contrary happens in the other half year, from July 12th to December 11th and this is what is understood when we say that the ascending node (denoted 8) of the sun's equator lies in 80° 21' longitude-a spot on the equator passing that node being then in the act of ascending from the southern to the northern side of the plane of the ecliptic-such being the conventional language of astronomers in speaking of these matters.

(392.) If the observations are made at other seasons (which, however, are the less favourable for this purpose the more remote they are from the epochs here assigned); when, moreover, as in strictness is necessary, the motion of the earth in the interval of the measures is allowed for (as for a change of the point of sight); the calculations requisite to deduce the situation of the axis in space, and the duration of the revolution around it, become much more intricate, and it would be beyond the scope of this work to enter into them. According to the best determinations we possess, the inclination of the sun's equator to the ecliptic is about 7° 20' (its nodes being as above stated), and the period of rotation 25 days 7 hours 48 minutes. †

*

See the theory in Lalande's Astronomy, art. 3258., and the formulæ of computation in a paper by Petersen Schumacher's Nachrichten, No. 419.

t Bianchi (Schumacher's Nach., 483,), agreeing with Laugier. Delambre makes it 25 Ob 17m; Petersen, 254 4h 30m. The inclination of the axis is uncertain to half a degree, and the node to several degrees. The continual changes in the spots themselves cause this uncertainty.

(393.) The region of the spots is confined, generally speaking, within about 25° on either side of the sun's equator: beyond 30° they are very rarely seen; in the polar regions, never. The actual equator of the sun is also less frequently visited by spots than the adjacent zones on either side, and a very material difference in their frequency and magnitude subsists in its northern and southern hemisphere, those on the northern preponderating in both respects. The zone comprised between the 11th and 15th degree to the northward of the equator is particularly fertile in large and durable spots. These circumstances, as well as the frequent occurrence of a more or less regular arrangement of the spots, when numerous, in the manner of belts parallel to the equator, point evidently to physical peculiarities in certain parts of the sun's body more favourable than in others to the production of the spots, on the one hand; and on the other, to a general influence of its rotation on its axis as a determining cause of their distribution and arrangement, and would appear indicative of a system of movements in the fluids which constitute its luminous surface bearing no remote analogy to our trade winds-from whatever cause arising. (See art. 239. et seq.) (394.) The duration of individual spots is commonly not great; some are formed and disappear within the limit of a single transit across the disc-but such are for the most part small and insignificant. Frequently they make one or two revolutions, being recognized at their reappearance by their situation with respect to the equator, their configurations inter se, their size, or other peculiarities, as well as by the interval elapsing between their disappearance at one limb and reappearance on the other. In a few rare cases, however, they have been watched round many revolutions. The great spot of 1779 appeared during six months, and one and the same groupe was observed in 1840 by Schwabe to return eight times. * It has been surmised, with considerable apparent probability, that some spots, at least, are generated again

Schum. Nach. No. 418. p. 150. The recent papers of Biela, Capocci, Schwabe, Pastorff, and Schmidt, in that collection, will be found highly interesting.

and again, at distant intervals of time, over the same identical points of the sun's body (as hurricanes, for example, are known to affect given localities on the earth's surface, and to pursue definite tracks). The uncertainty which still prevails with respect to the exact duration of its rotation renders it very difficult to obtain convincing evidence of this; nor, indeed, can it be expected, until by bringing together into one connected view the recorded state of the sun's surface during a very long period of time, and comparing together remarkable spots which have appeared on the same parallel, some precise periodic time shall be found which shall exactly conciliate numerous and well-characterized appearances. The enquiry is one of singular interest, as there can be no reasonable doubt that the supply of light and heat afforded to our globe stands in intimate connexion with those processes which are taking place on the solar surface, and to which the spots in some way or other owe their origin.

(395.) Above the luminous surface of the sun, and the region in which the spots reside, there are strong indications of the existence of a gaseous atmosphere having a somewhat imperfect transparency. When the whole disc of the sun is seen at once through a telescope magnifying moderately enough to allow it, and with a darkening glass such as to suffer it to be contemplated with perfect comfort, it is very evident that the borders of the disc are much less luminous than the centre. That this is no illusion is shown by projecting the sun's image undarkened and moderately magnified, so as to occupy a circle two or three inches in diameter, on a sheet of white paper, taking care to have it well in focus, when the same appearance will be observed. This can only arise from the circumferential rays having undergone the absorptive action of a much greater thickness of some imperfectly transparent envelope (due to greater obliquity of their passage through it) than the central.-But a still more convincing and indeed decisive evidence is offered by the phænomena attending a total eclipse of the sun. Such eclipses (as will be shown hereafter) are produced by the interposition of the dark body of the moon between the earth and sun,

the moon being large enough to cover and surpass, by a very small breadth, the whole disc of the sun. Now when this takes place, were there no vaporous atmosphere capable of reflecting any light about the sun, the sky ought to appear totally dark, since (as will hereafter abundantly appear) there is not the smallest reason for believing the moon to have any atmosphere capable of doing so. So far, however, is this from being the case, that a bright ring or corona of light is seen, fading gradually away, as represented in Pl. I. fig 3., which (in cases where the moon is not centrally superposed on the sun) is observed to be concentric with the latter, not the former body. This corona was beautifully seen in the eclipse of July 7. 1842, and with this most remarkable addition witnessed by every spectator in Pavia, Milan, Vienna, and elsewhere: three distinct and very conspicuous rose-coloured protuberances (as represented in the figure cited) were seen to project beyond the dark limb of the moon, likened by some to flames, by others to mountains, but which their enormous magnitude (for to have been seen at all by the naked eye their height must have exceeded 40,000 miles), and their faint degree of illumination, clearly prove to have been cloudy masses of the most excessive tenuity, and which doubtless owed their support, and probably their existence, to such an atmosphere as we are now speaking of.

(396.) That the temperature at the visible surface of the sun cannot be otherwise than very elevated, much more so than any artificial heat produced in our furnaces, or by chemical or galvanic processes, we have indications of several distinct kinds: 1st, From the law of decrease of radiant heat and light, which, being inversely as the squares of the distances, it follows, that the heat received on a given area exposed at the distance of the earth, and on an equal area at the visible surface of the sun, must be in the proportion of the area of the sky occupied by the sun's apparent disc to the whole hemisphere, or as 1 to about 300000. A far less intensity of solar radiation, collected in the focus of a burning glass, suffices to dissipate gold and platina in vapour. 2dly, From the facility with which the calorific rays of the sun traverse glass,

a property which is found to belong to the heat of artificial fires in the direct proportion of their intensity.* 3dly, From the fact, that the most vivid flames disappear, and the most intensely ignited solids appear only as black spots on the disc of the sun when held between it and the eye.† From the last remark it follows, that the body of the sun, however dark it may appear when seen through its spots, may, nevertheless, be in a state of most intense ignition. It does not, however, follow of necessity that it must be so. The contrary is at least physically possible. A perfectly reflective canopy would effectually defend it from the radiation of the luminous regions above its atmosphere, and no heat would be conducted downwards through a gaseous medium increasing rapidly in density. That the penumbral clouds are highly reflective, the fact of their visibility in such a situation can leave no doubt.

(397.) As the magnitude of the sun has been measured, and (as we shall hereafter see) its weight, or quantity of ponderable matter, ascertained, so also attempts have been made, and not wholly without success, from the heat actually communicated by its rays to given surfaces of material bodies exposed to their vertical action on the earth's surface, to estimate the total expenditure of heat by that luminary in a given time. The result of such experiments has been thus announced. Supposing a cylinder of ice 45 miles in diameter, to be continually darted into the sun with the velocity of light, and that the water produced by its fusion were continually carried off, the heat now given off constantly by radiation would then be wholly expended in its liquefaction, on the one hand, so as to leave no radiant surplus; while, on the other, the actual temperature at its surface would undergo no diminution.

(398.) This immense escape of heat by radiation, we may remark, will fully explain the constant state of tumultuous

By direct measurement with the actinometer, I find that out of 1000 calorific solar rays, 816 penetrate a sheet of plate glass 0.12 inch thick; and that of 1000 rays which have passed through one such plate, 859 are capable of passing through another. H. 1827.

The ball of ignited quicklime, in Lieutenant Drummond's oxy-hydrogen lamp, gives the nearest imitation of the solar splendour which has yet been produced. The apppearance of this against the sun was, however, as described in an imperfect trial at which I was present. The experiment ought to be repeated under favourable circumstances. Note to the ed. of 1833.

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