that the appearances of the spots are occasioned by real excavations in the solar globe. He supposed the sun to consist of a dark nucleus, covered only to a certain depth by a luminous matter, not fluid, through which openings are occasionally made by volcanic or other energies, permitting the solid nucleus of the sun to be seen ; and that the umbra which surrounds the spot is occasioned by a partial admission of the light upon the shelving sides of the boundary opposite to the observer. It is evident that, in proportion as these excavations are seen obliquely, their apparent dimensions will be diminished ; one of the edges will disappear as it approaches the sun s limb, or come more into view as it advances towards the middle of the disk ; when the spot is about to leave the disk, the bottom of the excavation, or the nucleus seen through it, will first disappear, but a sort of faint or obscure spot will remain visible as long as the visual ray penetrates the
cavity.Dr Wilson s theory was keenly combated by Lalande, who adduced several observations of his own, and some by Cassini, that could not be explained by means of it ; and urged with reason that an hypothesis, founded on the uniformity of appearances which in reality are exceedingly variable, was entitled to little consideration. Lalande himself supposed the spots to be scoriae which have settled or fixed themselves on the summits of the solar mountains ; an opinion which he grounded on the circumstance that some large spots which had disappeared for several years were observed to form themselves again at the identical points at which they had vanished.
Sir William Herschel, with a view to ascertain more accurately the nature of the sun, made frequent observations upon it from the year 1779 to the year 1794. He imagined the dark spots on the sun to be mountains, which, considering the great attraction exerted by the sun upon bodies placed at its surface, and the slow revolution it has upon its axis, he thought might be more than 300 miles high, and yet stand very firmly. He says that in August 1792 he examined the sun with several powers from 90 to 500, when it evidently appeared that the dark spots are the opaque ground or body of the sun, and that the luminous part is an atmosphere, through which, when interrupted or broken, we obtain a view of the sun itself. Hence he concluded that the sun has a very extensive atmosphere, consisting of elastic fluids that are more or less lucid and transparent, and of which the lucid ones furnish us with light. This atmosphere, he thought, can not be less than 1843 nor more than 2765 miles in height; and he supposed that the density of the luminous solar clouds needs not be much more than that of our aurora borealis, in order to produce the effects with which we are acquainted. The sun, then, if this hypothesis be admitted, " is similar to the other globes of the solar system with regard to its solidity, its atmosphere, its surface diversified with mountains and valleys, its rotation on its axis, and the fall of heavy bodies on its surface ; it therefore appears to be a very eminent, large, and lucid planet, the primary one in our system, disseminating its light and heat to ail the bodies with which it is con nected."
Herschel supposed that there are two regions or strata of solar clouds ; that the inferior stratum is opaque, and probably not unlike our own atmosphere, while the superior is the repository of light, which it darts forth in vast quantities in all directions. The inferior clouds act as a curtain to screen the body of the sun from the intense brilliancy and heat of the superior regions, and, by reflecting back nearly one-half of the rays which they receive from the luminous clouds, contribute also greatly to increase the quantity of light which the latter send forth into space, and thereby perform an important function in the economy of the solar system. The luminous clouds prevent us in general from seeing the solid nucleus of the sun; but in order to account for the spots, he supposes an empyreal elastic gas to be constantly forming at the surface, which, carried upwards by reason of its inferior density, forces its way through the planetary or lower clouds, and mixing itself with the gases which have their residence in the superior stratum, causes decomposition of the luminous matter, and gives rise to those appearances which he describes under the name of corrugations. Through the openings made by this accidental removal of the luminous clouds, the solid body of the sun becomes visible, which, not being lucid, gives the appearance of the dark spots or nuclei seen through the telescope. The length of time during which the spots continue visible renders it evident that the luminous matter of the sun cannot be of a liquid or gaseous nature ; for, in either case, the vacuity made up by its accidental removal would instantly be filled up, and the uniformity of appearance invariably maintained.
But, perhaps, the most important of all the discoveries which have been made respecting the sun spots, are those which relate to the variation of these objects in number, and in the amount of solar area which they cover. We owe the initiation of observations on these points to Schwabe of Dessau. They were commenced in the year 1826, and continued without intermission, except in one case, during the astronomer s illness, for forty-six years. Before many years had elapsed, Schwabe discovered that the spots wax and wane in frequency in a period of about eleven years. At the time of spot-minimum the sun remains often for several days not only clear of spots, but with a singular smoothness of aspect, even the minute mottlings ordinarily seen on his surface either passing away for a time or be coming less conspicuous than usual. From such an epoch there is a gradual return to the spotted condition, and usually in four or five years the maximum of spot-fre quency is reached ; then there is a more gradual reduction, until, in rather more than eleven years on the average, the minimum is reached. The table on page 787 indicates the facts actually observed by Schwabe between 1826 and 1868, supplemented by observations of a similar kind made at the Kew Observatory to the year 1871.|1}}
It will be seen that the law of variation is not uniformly periodic ; the intervals between maximum and maximum are not strictly equal, and the maxima are neither equal nor similar.
Stewart and Loewy have indicated further laws of variation, apparently connecting the changes of spot-frequency with the movements of the planets Mercury, Venus, the earth, and Jupiter. According to their views the great eleven-year period would correspond with the periodic re volution of Jupiter. It is, however, noteworthy that Pro fessor Wolf of Zurich estimates the spot period at 11 11 years, a period differing too much from Jupiter s to accord with De la E,ue s theory. And Carringtun notes that, whereas certain epochs of spot-maxima accord with the passage by Jupiter of his perihelion, others accord almost as exactly with Jupiter s aphelion passage. Yet Wolf accepts the theory of planetary influence. " He finds," says Sir John Herschel, " that a perceptibly greater apparent activity prevails annually, on the average, in the months of September to January, than in the other months of the year; and, again, by projecting all the results in a continu ous curve, he finds a series of small undulations succeed ing each other at an average interval of 7 65 months, or 637 of a year. Now the periodic time of Venus, re
duced to a fraction of a year, is 616, a coincidence cer-