neous movements.

CHAP. XIV. sively in some portions. What we shall see will be this. The Simulta portion of the prominence at rest will give us no alteration of wave-length; its bright line will be in a line with the corresponding black one in the spectrum. The portion moving towards the eye, however, will give us an alteration of wave-length towards the violet. You are now in a position to grasp the phenomena revealed to me by my spectroscope on the 12th instant, when at times the F line was triple! the extreme alteration of wave-length being such that the motion of that part of the prominence giving the most extreme alteration of wave-length must have exceeded 120 miles per second, if we are to explain these

Connection

FIG. 99.-Solar cyclone. Left-hand diagram, retreating side of cyclone on slit; centre diagram, both sides on slit; right-hand diagram, advancing side on slit.

phenomena by the only known possible cause which is open to us.

By moving the slit it was possible to see in which part of the prominence these great motions arose, and to follow the change of wave-length to its extremest limit.

By the kindness of Dr. Balfour Stewart, I am able to of spectro exhibit to you some of the Kew sun-pictures which show scopic and telescopic you how these spectroscopic changes are sometimes conchanges. nected with telescopic ones.

On the 21st April there was a spot very near the limb which I was enabled to observe continuously for some time. At 7.30 A. M. there was a prominence visible in the field of view, in which tremendous action was evidently

going on, for the C, D, and F lines were magnificently CHAP. XIV. bright in the ordinary spectrum itself, and as the spotspectrum was also visible, it was seen that the prominence was in advance of the spot. The injection into the chromosphere surpassed anything I had seen before, for there was a magnesium cloud quite separated from the limb, and high up in the prominence itself.

By 8.30 the action had quieted down, but at 9.30 another throb was observed, and the new prominence was moving

A magne sium cloud above the

observed

chromo

sphere.

FIG. 100.-Non-coincidence of bright and dark F line when tangential slit is used, and when both photospheric and chromospheric light is admitted.

away with tremendous velocity. While this was going on, the hydrogen lines suddenly became bright on the other side (the earth's side) of the spot, and widened out considerably-indeed to such an extent that I attributed their action to a cyclone, although, as you know, this was a doubtful case.

Now, what said the photographic record? The sun was photographed at Ioh. 55m. A.M., and I hope you will be able to see on the screen how the sun's surface was disturbed near the spot. A subsequent photograph at 4h. Im. P.M. on the same day shows the limb to be actually broken in that particular place: the photosphere seems to have

The limb of the sun broken in the photograph at

the place where the cyclone was observed.

CHAP. XIV. been absolutely torn away behind the spot, exactly where the spectroscope had afforded me possible evidence of a cyclone!

In connection with the last branches of the research I have brought to your notice, I may remark that we have two very carefully prepared recent maps of the solar spectrum, one by Kirchhoff, the other by Ångström, made a few years apart and at different epochs with regard to the sun-spot period. If you look at these maps, you will see a vast difference in the relative thicknesses of the C and F lines, and great differences in the relative darkness

Kirchhoff

Ångstrom

FIG. 101-Comparison of b and adjacent lines, as mapped by Kirchhoff and Ångström. and position of the lines; and if I had time, I could show you that we now may be supplied with a barometer, so to speak, to measure the varying pressures in the solar and stellar chromospheres; for, depend upon it, every star has, has had, or will have, a chromosphere, and there are no such things as "worlds without hydrogen," any more than Aplica there are stars without photospheres. I suggested in 1866 tion of the that possibly a spectroscopic examination of the sun's results of solar study limb might teach us somewhat of the outburst of the star to the fixed in Corona, and already we see that all that is necessary stars. to get just such an outburst in our own sun is to increase

the power of his convection currents, which we know to CHAP. XIV. be ever at work. Here, then, is one cataclysm the less in astronomy-one less "World on Fire," and possibly also

a bright light thrown on the past history of our own. planet.

tion to variable

stars.

I might show you further that we now are beginning to have a better hold on the strange phenomena presented by variable stars, and that an application of the facts I have Applica brought to your notice this evening, taken in connection with the various types of stars1 which have been indicated by Father Secchi with admirable philosophy, opens out generalizations of the highest interest and importance; and that, having at length fairly grappled with some of the phenomena of the nearest star, we may soon hope for more certain knowledge of the distant ones.

At present, however, we may well leave speculation for those who prefer it to acquiring facts; let us rather, emboldened by the work which this new method of research has enabled us to accomplish in this country, under the worst atmospheric conditions, in seven short months, go on quietly deciphering one by one the letters of this strange. hieroglyphic language which the spectroscope has revealed to us a language written in fire on that grand orb which to us earth-dwellers is the fountain of light and heat, and even of life itself.

When this was written I did not know that Mr. Rutherford was the first to suggest this field of research.

THE AMERICAN ECLIPSE, 1869.

I. FIRST IMPRESSIONS.

CHAP. XV. IF Our American cousins in general hesitate to visit our little island, lest, as some of them have put it, they should fall over the edge; those more astronomically inclined may very fairly decline, on the ground that it is a spot where the sun steadily refuses to be eclipsed. This is the more tantalizing, because the Americans have just observed their third eclipse this century, and already I have been invited to another, which will be visible in Colorado, four days' journey from Boston (I suppose I am right in reckoning from Boston?) on July 29, 1878.

Thanks to the accounts in Silliman's Journal and the Philosophical Magazine, and to the kindness of Professors Winlock and Morton, who have sent me some exquisite photographs, I have a sufficient idea of the observations of this third eclipse, which happened on the 7th of August last, to make me anxious to know very much more about them an idea sufficient also, I think, to justify some remarks on what we already know.

A few words are necessary to show the work that had to be done.

An eclipse of the sun, so beautiful and yet so terrible to the mass of mankind, is of especial value to the astronomer,