378 COSMOS. 



observed by La Hire in 70 North Latitude, must be regarded 

 as a very rare phenomenon. 



This distribution of spots on the Sun's disc, their rarity 

 under the equator and in the polar regions, and their parallel 

 position in reference to the equator, led Sir John Herschel to 

 the conjecture, that the obstructions which the third vaporous 

 external atmosphere may present at some points to the libe- 

 ration of heat, generates currents in the Sun's atmosphere 

 from the poles towards the equator, similar to those which 

 upon the Earth occasion the trade-winds and calms near the 

 equator, owing to differences of velocity in each of the parallel 

 zones. Some spots are of so permanent a character, that 

 they have continued to appear for fully six months, as was 

 the case with the large spot visible in 1779. Schwabe was 

 enabled to follow the same group eight times in the year 

 1840. A black nucleoid spot, delineated in Sir John Her- 

 schel' s Observations at the Cape (to which I have made such 

 constant reference), was found by accurate measurement to 

 be so large, that supposing the whole of our Earth to be 

 propelled through the opening of the photosphere, there 

 would still have remained a free space on either side of more 

 than 920 geographical miles. Sommering directs attention 

 to the fact, that there are certain meridian belts on the Sun's 

 disc, in which he had never observed a solar-spot for many 

 years together. (Thilo. de Soils maculis a Scemmeringio 

 observatts, 1828, p. 22.) The great differences presented in 

 the data given for the period of revolution of the Sun, are not, 

 by any means, to be ascribed solely to want of accuracy in the 

 observations; they depend upon the property exhibited by 

 some spots, of changing their position on the disc. Laugier has 

 devoted special attention to this subject, and has observed 

 spots which would give separate rotations of 24d. 28m. and 

 26d. 46m. Our knowledge of the actual period of the rota- 

 tion of the Sun, can therefore only be regarded as the mean 



