But, on the whole, it seems probable that pressure is chiefly in question, while it may be regarded as absolutely certain that temperature alone is insufficient to account for the observed change. Now, whether we regard the glowing hydrogen of the chromatosphere as forming a true constituent of the solar atmosphere or as bearing a somewhat similar relation to that atmosphere that the aqueous vapour in our own air bears to the permanent constituents, it is yet certain that the pressure of the hydrogen near the solar photosphere is a measure of the atmospheric pressure there. And from the observed width of the F-line near the Sun's' limb (see fig. 72) it has been estimated by Wüllner that the pressure on the base of the chromatosphere, or at the surface of the photosphere, is below the pressure of the Earth's atmosphere.'* He has even assigned the

tinguish between the two effects which play the chief part in solar phenomena.

The disappearance of one of the magnesium lines at a certain height above the spectrum of the limb (see fig. 71) in which the lines в belong to magnesium, is significant of a very low degree of pressure at the level where the shortest line ceases to be visible.

* It must not be forgotten, however, that the width of the hydrogenline where it actually reaches the spectrum of the limb is not known. The observed width on which Wüllner founded his researches may be that corresponding to a height of 50, 100, or even 200 miles above the photosphere; and within these 50, or 100, or 200 miles an increase of pressure may take place by which the actual density of the atmosphere close by the photosphere may be enormously increased. There may be an atmosphere including the vapours of iron. sodium, magnesium, &c. (of all the elements, in fine, whose dark lines appear in the solar spectrum), extending, say, 100 miles above the photosphere; and yet no instruments we possess could suffice to reveal any trace of its existence, unless the dark lines in the solar spectrum be thought to demonstrate the fact that such an atmosphere actually does exist. The

limits of pressure at the level of the solar photosphere as lying certainly between 50 and 500 millimetres (or between 2 inches and 20) of a mercurial barometer at the Earth's surface.'

It is within the sierra and certain of the prominences that spectroscopes of high dispersive power exhibit those signs of cyclonic motions taking place in the solar atmosphere, the recognition of which has seemed so surprising and inexplicable. The study of the last part of Chapter III. will have shown the reader that if very rapid motions are taking place, either due to the swift rush of the glowing hydrogen through the solar atmosphere or to the effects of cyclonic motions in the atmosphere itself, by which the glowing hydrogen is borne along, the spectroscope-if only its dispersive power be sufficient cannot fail to give unmistakable

fact that on one occasion Mr. Lockyer has seen hundreds of the Fraunhofer lines (as bright lines) in the spectrum of the chromatosphere renders this far from improbable. The arguments on the strength of which it has been assumed that the absorption to which the dark lines are due takes place below the visible photosphere, appear, to say the least, far from demonstrative.

[I have preferred to leave this note precisely as it appeared in the first edition, save for the correction of one clerical error. What I have here suggested as probable was established by an observation made by Prof. Young, of America, during the eclipse of December 1870. The slit of the spectroscope being so placed as to include the part of the Sun's edge which was last covered by the Moon, included, necessarily, the lowest layers of the solar atmosphere. So long as the Sun's direct light illuminated the spectroscopic field only, a few bright lines belonging to the sierra could be seen. But so soon as the last fine sickle of the Sun's disc was concealed, hundreds of bright lines made their appearance, and continued visible for a few seconds, -in fact until the Moon had covered the region thus shown to form a true atmosphere, exceedingly complex, and formed in large part of the glowing vapours our familiar gases.]

of

evidence on the point. The problem is altogether simpler and easier indeed than that which Dr. Huggins had to deal with when he undertook to measure the velocity with which Sirius is winging its flight through space. For there in the solar spectrum are the very lines of hydrogen with which the chromatospheric lines

are to be compared. If the spectroscopic dispersion suffice, the least experienced observer can tell as certainly that a solar storm is in progress as the terrestrial observer can tell by the motions of cirrus clouds that the upper regions of our own atmosphere are disturbed. The accompanying illustration (fig. 73), for instance, shows how the F-line in the chromatospherespectrum is at times swayed (as it were) from coincidence with the dark F-line of the ordinary solar spectrum. At 1 we see the line deflected towards the violet, showing that the portion of the chromatosphere under examination was moving rapidly towards the observer; at 2 we see a deflection both towards the

red and towards the violet, indicating that in the same field of view (that is, in the portion of the chromatosphere included within the slit) there were masses moving towards as well as from the eye; while, lastly, at 3 we see a deflection towards the red, indicating a rapid motion from the eye. During some observations such as these, Mr. Lockyer has had evidence of motions at the almost inconceivable velocity of 120 miles per second. I cannot, however, accept his conclusions as to the distribution of the motions over the Sun's surface, because he seems to take very little account of what certainly is the fact, that the extension of the portion of chromatosphere under examination is very great in the direction of the line of sight. Asuming an apparent depth of only ten seconds (which s within the usually observed limits), a tangent-line to the Sun's surface passes through a range of upwards of 60,000 miles; and to speak of the clear recognition of a solar cyclonic storm only 1,500 miles in diameter (as described by Mr. Lockyer) when the visual ray passes through a depth forty times as great, seems to me wholly inadmissible. We have no evidence that the portions of the chromatosphere giving the three displacements shown in fig. 73 may not have lain 20,000, 30,000, or even 40,000 miles apart, in the direction of the line of view; in other words, that

* One may notice in some papers on this subject a mode of explaining the observed phenomena which seems to imply that the spectroscope exhibits to us the coLdition of a mere slice of the solar envelopes. The effect of the Sun's solidity in giving an enormous extension to the visual lines through the chromatosphere seems wholly neglected even

the observed solar storms, though undoubtedly raging with amazing fierceness, were necessarily cyclonic in character. This remark does not apply with equal force to the evidence deduced from the appearance of the prominence F-line depicted in fig. 74, because we

cannot consider it likely that two prominences of the enormous height indicated by the extension of the shattered F lines from the spectrum of the limb would lie along the same visual line at the moment of observation.

Professor Young, of America, has, however, noted

where a reference to the breadth of the region really included within the slit would appear to invite a reference to the other form of extension.

In this figure the dots below the spectra indicate the amount of displacement on either side of the normal position of the F-line, corresponding to a velocity of 8, 16, and 28 German geographical miles per second (a German geographical mile being equal in length to a fifteenth part of a degree of the Earth's equator-i.e., to about 4 English miles).