*

picture. The reader will be prepared to examine with so much the greater confidence the interesting picture of a spot with the surrounding faculæ (fig. 55), drawn by M. Chacornac. He will recognise also the close

correspondence between the appearance of the region around this spot and Sir William Herschel's description of the faculæ (p. 180).

* Unfortunately such extreme accuracy of delineation is not ordinarily to be met with among solar observers.

A

We owe to Mr. Dawes the detection in certain spots of a rotatory motion, as though these regions were the scene of some tremendous solar tornado. spot of enormous dimensions was observed by him to have rotated through half a complete circuit in the course of about six days. Other spots have exhibited an even more rapid motion, and the spectroscopic

observations made by Mr. Lockyer on parts of the Sun near the limb (where such cyclonic motions would necessarily involve a rapid motion towards and from the eye) seem to place beyond question the existence of solar tornadoes having a velocity of 40 or 50, in some cases even 120 miles per second. So that we need no longer regard (with Spörer) the whirling

appearance noticed in the accompanying drawing by Fr. Secchi (fig. 56) as due to optical illusion.

The immense dimensions of some spots well deserve thoughtful consideration. When we remember that the least spot which could be perceived with the most powerful telescope must have an area of at least 50,000 miles, it will be understood how enormous these spots must be which have been distinctly visible to the unaided eye.* But we have trustworthy measurements to refer to in this matter. Pastorff, in 1828, measured a spot whose umbra had an extent four times greater than the Earth's surface. In August, 1859, a spot was measured by Newall which had a diameter of 58,000 miles-that is, exceeding more than seven times the diameter of our Earth. But spots of even greater dimensions have been observed. In June, 1843, a spot was visible, which, according to Schwabe's measurements, had a length of no less than 74,816 miles. On March 15, 1858, observers of the great eclipse had the good fortune to witness the passage of the Moon over a spot which had a breadth of 107,520 miles. It was in the same year that the largest spot of any whose records have been handed down to us, was visible upon the solar disc. It had a breadth of more than 143,500 miles; so that across it no less than eighteen globes as large as our Earth might have been placed side by side. At a very moderate computation of the depth of this solar cavity, it may

*Of the groups shown in Plate II. three were visible to the naked eye, the largest of them (near the centre) being quite conspicuous.

be assumed that the mass of 100 earths such as ours would barely have sufficed to fill it to the level of the solar photosphere.

During the past two years many spots and groups of enormous extent have been noticed. Those shown in Plate II. may be cited as instances; but others fully as large have lately been observed.

The rapidity with which some spots have changed in figure, or even wholly disappeared, would be wholly incredible were it not that astronomers of the highest repute for accuracy have supplied the records of such changes. Dr. Wollaston says:-'I once saw with a 12-inch reflector a spot which burst in pieces as I was looking at it. I could not expect such an event, and therefore cannot be certain of the exact particulars; but the appearance, as it struck me at the time, was like that of a piece of ice when dashed on a frozen pond, which breaks in pieces and slides on the surface in various directions.'* Biela also notes that spots disappear sometimes almost in a single moment. converse of such a change has been witnessed by Krone, who observed a spot of no inconsiderable dimensions which sprang into existence in less than a minute of time. On one occasion a momentary distraction caused Sir William Herschel to turn away his

The

* Of course this description refers only to the appearance which the spots ordinarily present, of being real bodies rather than openings. What Wollaston has described as the breaking up of a spot into pieces must in reality be looked upon in all probability as the sudden change of a single whirlstorm into a number of smaller ones.

eyes from a group of spots he was observing. When he looked again the group had vanished!

In this place I shall not enter into the discussion of the nature of the spots-a matter, indeed, on which at present it is very difficult to form an opinion. It has been my purpose in this chapter to consider rather the evidence which has been adduced respecting the solar surface than the physical theories put forward in explanation of that evidence.

Two points remain, however, to be briefly touched

upon.

The first is the evidence supplied by the spectroscope respecting the spots. It is evident that by bringing a spot under the slit of the spectroscope in the way described at the close of the preceding chapter (see fig. 35) it becomes possible to institute a direct comparison between the spectrum of the umbra, penumbra, and surrounding faculæ. If any lines belonging to the ordinary solar spectrum disappear in these regions, or if new lines make their appearance, we can at once become cognisant of the fact, because we see the spectra of these regions simultaneously. In like manner we can determine whether any change takes place in the character and appearance of any solar line-for instance, whether it is wider or narrower in the spectra of certain regions, or whether it changes into a bright line. Now, fig. 57 will illustrate the peculiarities which make their appearance when a spot is brought in this way under examination. Here the length of the spectrum (only