THE SUN’S CORONA. 
387 
There is, in fact, very little to choose between this form of 
the theory and the former. It will doubtless be deemed wholly 
unnecessary to discuss further a theory against which such 
decisive geometrical arguments may be adduced, especially as, 
according to its most ardent advocates, the theory has for its 
main support so vague a conception as that of 66 a possible 
action at the moon’s surface.” 
We are reduced, therefore, to accept the sole remaining 
theory that the corona is a solar appendage ; and our subject 
is proportionately enhanced in interest, since so viewed the 
corona becomes the most extensive region within the solar 
system. Granting to it no greater apparent extension than 
half a degree from the moon’s limb, it must yet have a diameter 
three times as great as the sun’s, and therefore a volume (en- 
closing his globe) twenty-seven times as great. But we may 
fairly assume that the greatest observed extension of the corona 
falls far short of its true boundary, since the state of the atmo- 
sphere tends importantly to affect the extent of the corona’s 
outline, always reducing it, but not always by the same amount. 
And since the corona has been observed to extend so far as 8 
degrees from the eclipsed moon, or to a distance exceeding some 
fifteen times the moon’s diameter, we should have to assign 
to the corona a volume exceeding about 30,000 times that of 
the sun. 
But it seems clear that we cannot regard the corona as a 
solar atmosphere. The arguments founded by Mr. Lockyer on 
the laboratory experiments of his eminent ally, Professor 
Frankland, seem to me to be wholly convincing on this point. 
The pressure of the solar atmosphere at the level of the sum- 
mits of the prominences must be exceedingly small, since, even 
near the base of the chromosphere, the pressure is not consi- 
derable. From the researches of Wullner it would seem that, 
near the base of the chromosphere, the pressure corresponds to 
a barometric height of between 50 and 500 millimetres — that 
is, roughly, between two and twenty inches.* Eighty or a 
hundred thousand miles or so above this level, the pressure 
* It seems to me open to doubt whether at the real base of the chromo- 
sphere — that is, at the very level where the photosphere and the chromosphere 
meet — the atmospheric pressure may not exceed, and that enormously (even 
many thousand fold), the pressure estimated from the observed width of the 
hydrogen lines at the apparent base of the chromosphere; for the most 
powerful telescope yet constructed could not recognise as a sensible quantity, 
a stratum next the photosphere of even 100 miles in thickness. Yet re- 
membering the compound character of the solar atmosphere at its lowest 
levels, it would be within such a stratum, that by far the greatest increase 
of pressure would take place. 
