72 COSMOS. 



equatorial region between 3° north and b^' south latitude, 

 and that they do not occur at all in the polar regions. They 

 are, on the whole, most frequent in the region between ll'^ 

 and 15° north of the equator, and generally of more common 

 occurrence in the northern hemisphere, or, as Sommering 

 maintams, may be seen there at a greater distance from the 

 equatorial regions than in the southern hemisphere. {^Out- 

 lines, ^ 393 ; Obse?"vatio7is at the Cape, p. 433.) Galileo 

 even estimated the extreme limits of northern and southern 

 heliocentric latitude at 29°. Sh John Herschel extends them 

 to 35°, as has also been done by Schwabe. (ScJuwi. Astr. 

 Nachr., No. 473.) Laugier found some spots as high as 41° 

 {Comptes Rendus, tom. xv., p. 944), and Schwabe even in 

 50°. The spot observed by La Hire in 70° north latitude, 

 must be regarded as a veiy rare phenomenon. 



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

 under the equator and in the polar regions, and their paral- 

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

 to the conjecture that the obstructions which the third vapor- 

 ous external atmosphere may present at some point-* to the 

 liberation of heat, generates currents in the Sun's atuiosphere 

 from the poles toward the equator similar to those which upon 

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

 tor, 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 Herschel's Ob- 

 ^ervations 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 re- 

 mained a free space on either side of more than 920 geograph- 

 ical miles. Sommering directs attention to the fact that there 

 are certain meridian belts on the Sun's disk in which he had 

 never observed a solar spot for many years together. ( Thilo. 

 de Solis maculis a Sai,mmeri7igio observatis, 1828, p. 22.) 

 The great differences presented in the data given for the pe- 

 riod of revolution of the Sun are not, by any means, to be as- 

 cribed solely to want of accuracy in the observations ; thoy 

 depend upon the property exhibited by some spots, of chang- 

 ing their position on the disk. Laugier has devoted special 

 attention to this suVject, and has observed spots which would 



