Solar Structure with Observation. 1938 
referred to these forms, and to my description of them. The 
are indeed so remarkable, and at first sight so apparently con- 
firmatory of the views alluded to, that only after long study 
have been led to think them not so much assimilable to the 
products of cooling upon a liquid surface as to certain cloud 
forms of our own atmosphere. 
To furnish material fora public examination of these details, 
whose study is so eminently instructive, it is necessary, as has 
been already remarked, (since photography cannot yet seize 
them,) to make drawings in which the single aim of the de- 
signer is to set down with a minute fidelity specific forms ; 
aiming, in short, much more to produce a piece of accurate 
topography than a picture; but while it is on studies made of 
this minute exactness that discussion will be most profitable, 
their reproduction for the press is a work of so much labor that 
this kind of illustration will probably remain unusual. 
The steel engraving, plate v1, from studies made at the Alle- 
gheny Observatory chiefly with the full aperture (13 English 
inches) of its equatorial, has been prepared by the kind further- 
ance of Prof. George F. Barker, of Philadelphia, its execution 
being secured at the hands of an engraver who has done his 
work with peculiar fidelity and skill. I trust it will be ac- 
cepted as a means of putting the reader in a certain sense in 
the place of the observer, and enabling him himself to directly 
compare theory with the facts of observation. This plate is 
made from sketches taken on the 28d, 24th and 25th of De- 
cember, 1873, of the eastern extremity of the great spot then 
nearing the center of the sun, and about 12° south of the solar 
equator. It is called a “ typical spot” because (since the details 
could not be completed at a sitting) it is less an accurate out- 
line of what could be seen at any one moment, than an assem- 
blage of the different types presented, in their proper connec: 
tion. The whole, then, is taken from observation; but while 
the details of the adjacent photosphere have been supplemented 
from other studies, everything in the main body of the spot is 
the most literal transcript I could make of specific penumbral 
and umbral forms. 
The sun had been hidden here for some days before the 23d 
of December, when the sky cleared, disclosing a spot of more 
than usual size. Although a daily record of the solar surface 
1S maintained at the Allegheny Observatory, the weather for 
some weeks before had interrupted it so capriciously that | am 
unable to say with certainty what the age of the spot was when 
it suddenly presented itself, but unquestionably it had at this 
time already passed through the initiatory stage of its forma- 
tion, and had entered upon that in which the forms seen earlier 
have commenced to become segmented or distorted, while still 
