PORTION OF THE COSMOS. THE SUN. 281 



which the third, or outermost, vaporous envelope may 

 oppose at some points to the escape of heat, may give rise 

 in the solar atmosphere to currents from the poles to the 

 equator, similar to those which, from the different velocity of 

 rotation under different parallels of latitude, cause on our 

 globe the trade-winds and the calms which prevail in 

 the more immediate vicinity of the equator. Particular 

 spots are sometimes so permanent as to return continually 

 for six entire months, as the large spot of 1779. Schwabe 

 was able to trace the same group eight times in the 

 year 1840. A black nucleus which is figured in the Cape 

 Observations of Sir John Herschel, (of which I have so ex- 

 tensively availed myself), was found by exact measurement 

 to be of such magnitude, that if our entire earth had been 

 thrown into the opening in the photosphere, there would 

 still have remained on either side a vacant space of more 

 than 920 geographical miles. Sommering calls attention 

 to the circumstance that there are certain meridians or bands 

 of longitude in which during many years he never saw a solar 

 spot. (Thilo de Solis maculis a Scemmeringio observatis, 

 ] 828, p. 22.) The very different periods of rotation which 

 have been assigned to the Sun are not by any means to be 

 attributed solely to inaccuracy of observation ; they proceed 

 from the circumstance that some spots change their places 

 upon the Sun's disk. Laugier has devoted a particular 

 examination to this subject, and has observed spots from 

 which rotations of 24'28 and 26*46 days might be severally 

 derived. Our knowledge of the actual time of the Sun's 

 rotation can, therefore, only be affirmed to correspond to 

 the mean result derived from a great number of observed 

 spots, which by the permanence of their form and the in- 



