44 
naked eye which one does not see in the photographs. 
consider the eclipse of 1871 first. 
We see in a moment that we have something here at the 
maximum sunspot period different from what we have had 
before. To compare it with the record of the preceding 
minima in 1867 and 1878. Instead of having streamers limited to 
We will 
the equator, they exist in high latitudes, and instead of having | 
them limited to four chief maxima, as we had in the year after 
the minimum in 1868, the energy is now so great that they 
practically extend to every part of the sun. 
The directions of the lines of force, as they may be called, 
are very various: there are straight rifts; there are curved 
boundaries ; here another streamer is curved bodily, and so we 
go on.. We must always remember that in this photograph 
what we see is, after all, a projection. We have the spherical 
moon in front of the spherical sun, from which these streamers 
project in all longitudes—some straight towards the earth, the 
tips of which are seen over or under the moon, some more side- 
ways from parts of the sun nearer to or further from the eye than 
the central section, so that the unravelling of the appearances is 
very difficult, especially if the eclipse happens when either the 
sun’s north or south pole is tipped towards us or away from us 
to the greatest extent. 
So much for 1871: in another eleven years we have 
another maximum—that of 1882—an eclipse seen in Egypt. In 
this case we find the activity more general than in the former 
one. The top and the bottom of the diagram represent the 
north and the south poles of the sun as before; but we see 
now that the streamers are more broken up, and furthermore 
that the rifts visible round the north and south pole at the 
previous maximum are entirely covered up—not that the rifts 
were not there, but that one could not see them in consequence 
of the extreme brilliancy of the streamers that were thrown 
towards the eye from the sun between us and the plane passing 
through the solar poles. 
Independently of that, it is easy to recognise that there is a 
tremendous family likeness between the photographs taken at 
both maxima, whereas there is the greatest possible difference 
between either of them and the drawings or photographs taken 
at the minimum sunspot period. If we accept that, that is a 
very great step gained. 
After Maximum 
We need not, after what has gone before, take up any more 
time, which is short, by discussing the gradual descent to mini- 
mum. I say the gradual descent because we know there are 
more years consumed in going from maximum to minimum than 
from minimum to maximum. 
Of course all the various energies slacken down, the mean 
latitude of the spots and metallic prominences still getting 
lower till they reach lat. 8° N. and S. ; then another series of 
spots breaks out in lat. 35° N. and S., and the whole story 
begins anew. 
Summary 
Now let us deal with the results we have arrived at. At the 
maximum period the continuous spectrum of the corona gives 
way alm)st entirely to a spectrum of bright lines. When I say 
gives way almost entirely, I mean so far as this: the striking 
thing when you observe the spectrum of the corona at the maxi- 
mum period is a series of brilliant lines, or of brilliant circles, 
according as we use a slit, or simply look through a 
prism, and the brilliancy of the spectrum seen between these 
lines or rings is small compared with the brilliancy of the 
lines or rings. That indicates that the temperature of the gases 
in the corona is greater than the temperature of the other sub- 
stances, and of course is very much higher than it is during the 
time of the minimum, when the gases do not make themselves 
visible, and, as I said before, the chief spectroscopic effect 
obtained is the continuous spectrum with dark lines here and 
there, showing that some part of the light is derived from cooled 
solid particles which can and do reflect light from the subjacent 
photosphere. 
To deal with results, and to bring them together as sharply as 
may be, we find, first, that the dimness of the light and absence 
of bright lines at the minimum shows that the outer atmosphere 
of the sun is cooler at the minimum sunspot period. When I 
was in America in 1878, at the period of minimum to which we 
have referred, I saw at once that the corona was not anything 
like so brilliant as I had seen it previously in 1871 in India. 
Eventually, when the observations came to be discussed, the con- 
NATURE 
| May 13, 1886 
clusion arrived at was that the brilliancy was not one-seventh of 
what it was at the previous maximum. There is a very con- 
siderable difference which no one can mistake who observes one 
eclipse after another. 
Secondly, when the corona is thus cooler, and therefore 
dimmer, an extension in the plane of the sun’s equator is seen. 
A question arises here whether this extension is not seen at the 
maximum because the eye is so much affected by the very 
brilliant corona? That is a subject which will require to be 
investigated in subsequent eclipses. 
Thirdly, there are plenty of minor prominences at the mini- 
mum sunspot period ; there are no spots, or very few. 
Fourthly, the lower temperature, and therefore relative 
quiescence, of the solar atmosphere seems to depend on the 
absence of spots. That is an important matter; and the point I 
wish to make is this: the quietude cannot depend on the absence 
of prominences, because they are there—not so many of them, 
but still some prominences. 
Fifthly, when the spots begin in these higher latitudes, 30° 
or 35°, as we have seen, we get the first brightening of the 
corona. 
Sixthly, the coronal streamers follow the spots—by which I 
mean that the cones and coronal streamers put on their greatest 
intensity according as the spots have moved nearer to the equator. 
When we have the minimum sunspot period, you can hardly 
call that equatorial extension a streamer at all, because it is 
so very dim ; and further, I take it, it is really of a different 
nature and origin. 
The Circulation in the Sun’s Atmosphere 
If we make an attempt to discuss the circulation of the atmo- 
sphere, a question which we acknowledge to be an extremely 
difficult one, we must bear in mind the enormous difference 
between solar and terrestrial conditions. When a portion of 
the earth’s surface is heated in a whole zone—as the equator 
is in the tropics—by the sun, you see the heat is outside, an 
ascending current is formed, and winds from north to south set 
in. For instance, if we consider the equator, and suppose the 
sun to be over it, we get the earth’s atmosphere over that region 
more highly heated than those parts of the atmosphere near 
either pole; and the result is, we get an indraught current in 
that way, both from the northern and southern hemispheres. In 
consequence of these two currents meeting and beginning their 
ascent at some distance from the equator, we get a belt of calms, 
of reduced pressure, and we get almost perpetual rains. 
Now, you see. that is all very well as a piece of terrestrial 
meteorology, but it is of no value to us from the solar point of 
view, unless it sets us thinking how very different the conditions 
are. : 
The sun cannot be heated from the outside. We have seen, 
in fact, that one chief point about the sun is that it is cooled 
on the outside; that masses of gas going up to tremendous 
altitudes eventually arrive where the atmosphere is cold and 
quiet, and where they again take on the solid or liquid forms, 
when they begin to go downagain. Now the sun, if it is heated 
at all, must be heated from the inside. What do I mean by the 
inside? J mean—seeing that the phenomena we have been dis- 
cussing in these lectures take place outside the photosphere—that 
the inside must be something below the Jevel of the photosphere. 
Now what form must that heat take? It will take, as undoubt- 
edly we see in the metallic prominences it does take, the form 
of the ejection of the tremendously brilliant and incandescent 
vapours. How are the-e produced? Something must produce 
them ; they do not ascend of their own sweet will, or they 
would not come up so locally as we see them. 
We get this fact most indisputably, which I hope I have been 
able to make quite clear, that these ascents of vapours from 
below the spot region always accompany the spots, and they 
always follow the spots in time. Then is it not reasonable to 
suppose they are produced by the spots? You remember I 
objected to the word ‘‘ eruption” in connection with these pro- 
minences. I do not so much object to the word “ explosion,” 
for I cannot understand how if you get twenty million tons of 
meteorites falling down in a particular latitude of the sun, and 
plunging into the photosphere—I do not understand how 
there must not follow after that the most gigantic and terrific 
explosion, driving heated gases many hundreds and thousands of 
miles into the upper air along the line of least resistance, and 
disturbing the photosphere for months afterwards. Now that 
really does seem to be the plain English of what happens. 
