1864. | Herscunn on the Solar Spots. 225 
from this latter movement, by tracing the apparent paths of individual 
spots across the disc, the time of that rotation was early concluded, 
approximately by Galileo himself, and with more exactness by the use 
of micrometric measures by his successors—as well as the position of 
its axis in space, or, which comes to the same thing, the inclination of 
its equator to the ecliptic and the longitude of its ascending and 
descending nodes, As regards these latter particulars, the results 
arrived at by various observers and computists, especially the more 
modern ones (Lalande, Fixlmillner, Bohm, Laugier, and Carrington), 
are in as good accordance as could be expected, and may be stated at 
7° 15’ for the inclination of the axis, and 73° 40’ for the longitude of 
the ascending node for 1850; so that the north pole of the sun’s axis 
points nearly to the star + Draconis, and the south to a Plutei. As 
regards the time of rotation, however, the disagreement is more con- 
siderable, for a reason which will presently appear. 
Few spots, and those only very large ones or groups of such, are 
permanent enough to be traced through more than one or two succes- 
sive revolutions of the sun. Instances of three or four returns are 
extremely rare. Schwabe, however, in 1840, saw the same spot eight 
times in the middle of its course over the disc, having made seven full 
revolutions between May 11 and November 16. Single spots, or 
small groups, undergo such changes in a few days as to be hardly 
recognizable, and many originate and die out during a single transit, 
The origin of a spot when it can be observed, is usually traceable to 
some of those minute pores, or dots, which stipple the sun’s surface, and 
which begin to increase, to assume an wmbral blackness, and acquire a 
visible and at first very irregular and changeable shape. It is not 
till it has attained some measurable size that a penumbra begins to be 
formed, a circumstance strongly favouring the origination of the spot in 
a disturbance from below, upward ;—vice versd, as the spots decay, they 
become bridged across, the umbree divide, diminish in size, and close up, 
leaving the penumbre, which by degrees also contract and disappear. 
The evanescence of a spot is usually more gradual than its formation. 
According to Professor Peters and Mr. Carrington, neighbouring 
groups of spots show a tendency to recede from one another. 
The changes undergone in a few hours by large spots, or among 
groups, are such as to alter visibly their shapes and relative situations, 
and from day to day to transform them entirely. Professor Wolf 
observed one on March 10, 1861, which in the short interval of 
1h. 17m. underwent, not merely visible but enormous changes, altering 
its whole aspect. And when it is remembered that a single second on 
the sun’s surface (seen from the earth) corresponds to 46. miles linear 
measure, and a square second (almost the minimum visibile) to upwards 
of 20,000 square miles, we need not to be told that such changes imply 
movements of a rapidity to which our fiercest hurricanes offer no ap- 
proach ; so that the term “ viscous,” which has been applied by some to 
the fluid in which the photosphere floats, is in the last degree map- 
propriate. Mr. Dawes and Mr. Birt have observed the umbre of spots 
in some cases to rotate as it were slowly on their centres. 
