Schwabe showed, as the result of nearly 40 year's observations, that the number of groups of sun-spots is not the same from year to year, but has a maximum about every • 10 or 11 years; and gen. Sabine recorded the wonderful fact that the various epochs of maximum spot frequency are also those of maximum disturbance of our earth's mag netism. Here, then, we have a very curious bond of union between the sun and the planets of our system.
It was next shown by Carrington that sun-spots have a proper motion of their own— those near the solar equator moving faster than those near the poles.
While spots are darker than the general surface of the sun, there are also frequently observed patches brighter than the general surface. These are called facule, and they generally accompany spots, most frequently in their wake; but they are only distinctly visible near the sun's limb, and lose their specific luminosity near the center of the sun's disk.
Another phenomenon connected with our luminary is not less remarkable than sun-spots. This is the red flames, or protuberances, which were first observed surround ing the sun's disk on the occasion of a total eclipse, but which by ingenious methods of observation can now be rendered visible even when the sun is not eclipsed. This col ored envelope, less brilliant than the photosphere, or light-giving surface, and a mean height of 5,000 or 6,000 miles, is known as the chromosphere, and seems to consist mainly of incandescent hydrogen, jets of which are seen at times to be projected to a height of 200,000 miles. The velocity of these movements has been calculated to exceed at times 120 mile a second. Above the chrOmosphere there is a far deeper layer of cooler, sub-incandescent gases, among which is an unknown substance, which chiefly composes the outer portion, and is apparently lighter than hydrogen. These gaseous envelopes, extending together to 300,000 or even 500,000 miles above the photosphere, are now believed to cause the appearance called the corona, or white halo, which is seen to surround the dark body of the moon during an eclipse of the sun.
If a spot be a hollow, as we have reason to suppose, it is only necessary to believe that there has been a descending current of this cold absorbing atmosphere to account for the want of luminosity. In like manner, on this hypothesis, a facula will be a portion of the luminous matter, which has been removed high up into the atmosphere, and which thus escapes the absorbing influence of the atmosphere. A spot may thus be supposed, to be produced by two currents—one ascending, and carrying the hot luminous matter up; the other descending, and carrying the cold atmosphere down. The photosphere is thus in a constant state of agitation like ebullition.
Spots and their accompanying faculm are an indication of the activity of those "con vection currents" by which heat is brought to the surface to replace what has been radiated off. Their temporary prevalence then is held to be a sign of more than ordinary solar activity or expenditure of energy. This is palpably manifested on our globe Ix the gieater magnetic disturbance that takes place when sun-spots are numerous, as before mentioned. The diurnal range of the magnet freely suspended in the Kew observatory, shows an unmistakable correspondence with the waxing and waning of the spotted area on the sun; and the frequency of aurora; is found to be in perfect cor respondence with prevalence of spots. It is natural to seek to connect cycles of weather with the spot-period of the sun, but nothing has yet been conclusively established. A. coincidence has seemingly been made out between the recurrence of famines in southern India through deficient rain and the period of minimum spots; but the coincidence does not hold for other localities. Attempts have even been made to bring commercial crises and other recurring events into the spot-cycle.
As to the cause of the periodicity of sun-spots, observations made at Kew seem to establish a connection between the behavior of spots and the proximity of prominent planets; and it may thus be found that the recurrence of certain planetary positions in some way determines the spot-period.