464 MR WILLIAM SWAN ON THE 
trusted, of very much greater extent ;”* and yet spots of such immense magnitude 
seldom last much longer than six weeks, so that their edges must often approach 
at the rate of 1000 miles a-day.+ 
Now, as the spots have been supposed to arise from upward currents causing 
apertures in the sun’s luminous atmosphere, I conceive the higher red prominences, 
or those which remain visible at the middle of the total phase of a central eclipse, 
may in like manner be formed (as represented at fand g, fig. 10) by the same, or 
similar currents, in the sun’s atmosphere, breaking through the envelope of cloud 
that surrounds him, bending back the edges of the apertures they have formed, 
and sometimes carrying up detached masses of cloud, such as that which was seen 
at the late eclipse. We may, however, suppose the envelope of cloud to be some- 
times simply raised (as at 4), without being broken through; and in that state it 
may form the conical prominences which were observed at the late eclipse. 
Since the prominences reflect, they must also absorb light;+ and thus, the hy- 
pothesis which has been proposed regarding them, asswimes the presence of an en- 
velope of cloud surrounding the sun’s luminous atmosphere, capable of absorbing part 
of his light, and subject to occasional interruptions of its continuity. 
If, then, such an envelope surrounds the sun, it will probably be connected 
with various solar phenomena. Let us now inquire whether any appearances 
presented by the sun afford additional evidence of its existence. 
2. On the Increased Brightness of the Corona in the neighbourhood of the Red Prominences. 
According to my observations of the late eclipse, the hook-shaped prominence 
was accompanied by increased brightness of the corona in its neighbourhood. 
(Plate XII.). Now this isa necessary consequence of the supposition, that the higher 
red prominences are the upturned edges of apertures in the envelope of cloud sur- 
rounding the sun; for the absorbent medium having been removed in forming the © 
apertures, the sun’s light ought to illuminate the corona more powerfully over an 
aperture than elsewhere. At the same time, it does not follow that all red pro- 
minences should be near bright portions of the corona; for a prominence may be 
formed by the cloudy stratum being simply raised, without being perforated. 
* Dr Witson of Glasgow, who, by observing a large solar spot, was led to the discovery that it 
was an aperture in the sun’s luminous atmosphere, estimates the depth of its nucleus as “ not less 
than a semidiameter of the earth below the level of the sun’s spherical surface.” See his highly in- 
teresting paper, Philosophical Transactions, vol. Ixiv., 1774. 
+ Herscuet’s Outlines of Astronomy, 1851, par. 386. 
+ In certain circumstances steam has a red colour. May not the rose colour of the prominences 
indicate a property of the vapour composing them analogous to that possessed by steam, or, if they 
consist of aqueous vapour, identical with it? Professor Forsrs, to whom we are indebted for our 
knowledge of the red colour of steam, regards it as the principal or only cause of the rosy tint 
observed in clouds.—See his interesting paper, Edinburgh Transactions, vol. xiv., p. 371. 

