Proceedings of the Polytechnic Association. 1073 



Solar Spots. 



/'The recent passage of a large spot across the sun's equator reminds 

 US that just one hundred years have elapsed since Dr. Alexander 

 Wilson, of Glasgow, after a careful series of observations, gave the 

 first plausible explanation of this class of phenomena. He found that 

 one side of the penubral fringe which surrounds a dark nucleus, form- 

 ing the center of a solar spot, became narrower on approaching the 

 central line of the sun's disk, and after passing it the same side of the 

 fringe was widened, while the opposite side was gradually reduced in 

 width. Hence, he inferred that these appearances indicated the for- 

 mation of a vast funnel-shaped cavity in the sun's envelop. Ten 

 years later, in 1779, Sir William Herschel investigated the pheno- 

 mena more fully, and from the character of these spots made some 

 deductions regarding the constitution of the sun, which have been 

 generally accepted until within a very recent period. He sujpposed 

 the sun to be mainly a dark mass, surrounded by two atmospheres ; 

 that nearest to the center being less luminous than the outer enve- 

 lop, which is enormous in depth, of great brilliancy, and the source 

 of all solar light and heat. When, therefore, the envelops were rent 

 asunder, a portion of the sun's body would be seen as the dark nucleus, 

 while in the penumbra would be revealed the shelving sides of the 

 inner envelop. After Mr. Dawes announced the discovery of a 

 rotary motion in sun spots, their action was assigned to causes simi- 

 lar to those which produced cyclones and whirlwinds m our own 

 atmosphere. 



The density of the sun is only one-fourth the density of the earth, 

 while gravity at the surface of the sun is nearly twenty-eight timea 

 greater than that force on the earth ; furthermore, the heat in the 

 exterior of the sun is so intense that iron, nickel, magnesium, and 

 other metals, which the spectroscope assures us form a part of it, are 

 maintained in a state of elastic vapors ; these and other considerations 

 lead us to infer that the actual diameter of the solid, or perhaps only 

 liquid, portion of our great central luminary is not more than one- 

 half of its apparent diameter, consequently the dark nucleus forming 

 the cente^iof a solar spot does not have the significance assigned to it 

 by the elder Herschel. Moreover, the existence of a separate envelop 

 between the photosphere and the body of the sun is not essential to 

 a satisfactory explanation of the changing aspect of a sun spot, since 

 the shading of the penumbra is found to result from streaks of light 

 falling from the more luminous into the less luminous portion below. 



[Inst.] 68 



