42 
NATURE 
[May 13, 1886 
would be very violently heated ; we probably might get some 
explosions, and as the result of these explosions some liquid iron 
might be carried with the liquid water which has entered into 
the liquid iron here and there. The metallic prominences 
always are close to spots. They almost always follow them like 
the facule. I might have told you, in fact, while talking of 
this, that of Ir00 cases in which spots and facula: have been 
observed together, in 581 the faculz were to the left of, or behind 
the spots. Only in 45 were they on the right or in front. 
shall see the importance of this by and by. 
If we can invariably, as we do, associate the descent of material 
which, though we do not see it falling, we know is there, and that 
it is relatively cool—7f we can as:ocia’e these descending absorption- 
phenomena with a subsequ nt upward splash, we must look upon 
the most intensely active prominences as being return upward 
currents, though in some cases it may be that what we see as 
the spectrum of a prominence at the limb is, in part, that of 
the vapour descending to form a spot. 
’ 
We 
The Sunspot Pertoa 
The next thing we have to do is to discuss the periodicity of 
the various solar phenomena, to which attention has already 
been directed. It is worth while again to refer to the two very 
interesting and important curves in which Prof. Spérer has 
recorded the results of his own observations. 
When the spots are at their fewest the small number we do 
see begin in a high latitude N. or S., from 30° to 35°; as the 
spots increase in number and activity we get, at the maximum 
sunspot period, the chief appearances observed in middle lati- 
tudes—about lat. 18° ; and then the mean latitude of the spot zone 
still gets lower and lower, until at the next sunspot minimum 
we get two systems of spots—one of them, lowest in latitude 
(about 8° N. and S.), ending the first cycle, and another in lati- 
tude 30° beginning the next. These are the salient features of 
the periodicity to which we have now to confine our attention. 
It has been previously pointed out that there are other 
periodicities with a much shorter period than eleven years ; 
certain changes are seen to occur among the quiet prominences. 
Still this is the main periodicity with which we are familiar on 
the sun ; and what we have now to do is to endeavour to see 
whether we can follow all the phenomena in their changes. 
The two last maxima occurred in the year 1871 and eleven 
years afterwards in 1882 and some time after that year. At tho-e 
times we got the greatest amount of spotted area and the most 
intense solar action. Similarly the two last periods of minimum 
activity were in 1867 and eleven years afterwards in 1878. 
Now, in order to investigate this question in the most satis- 
factory manner, I think, and I doubt not you will agree with 
me, that we should begin with the simplest case. 
The Minimum 
The simplest case is evidently that in which the sun is 
quietest. At first sight it may appear a little hazardous to talk 
about the sun being at its quietest; but we know, as a matter 
of fact, that there is a tremendous difference at different times 
in the solar activity along the lines to which reference has been 
made, 
But in the light of what has already been stated let us sup- 
pose the sun at its yuietest, what phenomena shall we see? 
There will be very few of the ordinary tree-like prominences 
anywhere on the sun, and especially will there be a dearth of 
them near the poles and near the equator. 
There will be faculee, but the facule will be dim ; they will 
not present the bright appearance they generally do, and what 
there are will be mostly confined to the regions of latitude com- 
prised between 20° N. and 20° S.. 
If by means of a spectroscope we attempt to determine the 
chemical materials in the chromosphere, we shall find just those 
five lines only to which we have referred in the spectrum as 
ordinarily visible—that is, four lines of hydrogen, and one line 
named Ds. 
Practically speaking, there will be no spots visible upon the 
disk ; the disk will appear to be perfectly pure, almost equally 
illuminated throughout, barring always the darkening towards 
the limb. 
As there are no spots, or only very small ones in high lati- 
tudes, there will be, we can easily understand from what has 
gone before, no metallic prominences whatever. The spectro- 
sscope searching right round the limb of the sun will gather no 
ndications of violent action—no region giving us many lines— 
nothing but that simple spectrum of hydrogen to which I have 
already referred. 
What, then, is the appearance put on by the corona if we 
can manage to get an idea of a corona at the minimum sunspot 
period? We see, the moment that question is suggested, how 
excessively important it is that all eclipses should be observed, 
whether they occur at the maximum or the minimum of the 
solar activity. Fortunately, since the year 1860 these wonder- 
ful phenomena have been observed with more or less diligence ; 
and since the year 1871—that is fifteen years ago now—with 
few exceptions, not only have those eclipses been observed by 
the eye with great care, but photographs of the extremest value 
have been obtained. 
Unfortunately, that first minimum to which I have referred— 
the minimum in 1867—took place practically before the general 
introduction of this perfect photographic record of eclipses ; 
and there is no good photograph extant of that eclipse; but 
fortunately, good photographs were secured of the eclipse of 
1878. Youcan imagine our American cousins did not let an 
opportunity like that of advancing knowledge slip; and the 
result was that the whole land along the line of totality 
bristled with telescopes and cameras, which did their work in 
an adinirable way. So that in the eclipse of 1878 we did get a 
photographic record, that is to say, an absolutely trustworthy 
record, of the appearance presented by the sun’s corona at the 
minimum sunspot period. If it were not so, I should have 
hesitated to show you the drawing made in 1867 ; but I think you 
will say, when I show you these records together, that the draw- 
ing in 1867 is so like the photograph taken in 1878 that on that 
ground alone it is worthy of extreme confidence ; and if we can 
accord such confidence to it, we arrive at the very important 
conclusion that at two different sunspot minima the appearances 
presented by the corona were very much alike indeed. 
At the minimum period the chief feature is a tremendous ex- 
tension of the corona in the direction of the solar equator. At 
both the poles, north and south, there is a wonderful curving 
Fic 
19.—Outcurv:ngs at the solar poles (: 878). 
right and left ; this outcurving having been one of the most 
exquisite things which it is possible to imagine. ’ 
There is further evidence indicating that the equatorial 
extension on the photograph may only after all have been a 
part of a much more extended phenomenon, one going to almost 
incredible distances—con-idering it as a solar atmosphere—from 
the sun itself. 
It has already been stated that at that eclipse one observer 
took extreme precautions to guard his eyes from being fatigued 
by the light of the inner corona, which sometimes is so bright 
that observers have mistaken it for the limb of the sun itself. 
What this gentleman, Prof. Newcomb did, was to erect a screen 
which covered the moon anda space 12’ high round it. The re- 
sult was, that as soon as he took his station at the commencement 
of totality, he saw a tremendous extension of the sun’s equator 
on both sides the dark moon, the extension being greater than 
that recorded in the photograph. It does not follow that the 
photograph gives us the totality of the extension; it may be 
that the extended portions may have been so delicately illumin- 
ated, that they would not impress their image on the photo- 
graphic plate in the time during which that plate was exposed, 
or that the light itself is poor in blue rays. _So considerable was 
this extension, amounting to six or seven diameters of the dark 
moon, which practically may be taken to be the same as that of 
the sun behind it, that Prof. Newcomb had at once suggested to 
him the zodiacal light. It happened that while this eclipse was 
