100 Dr. J. R. Mayer on Celestial Dynamics. 



the beginning of this chapter, which assumes that the sun, like 

 the ocean, is constantly losing and receiving equal quantities of 

 matter. This harmonizes with the supposition that the vis viva 

 of the universe is a constant quantity. 



VII. The Spots on the Sun's Disc. 



The solar disc presents, according to Sir John Herschel, the 

 following appearance. " When the sun is observed through a 

 powerful telescope provided with coloured glasses in order to 

 lessen the heat and brightness which would be hurtful to the 

 eyes, large dark spots are often seen surrounded by edges which 

 are not quite so dark as the spots themselves, and which are 

 called penumbra3. These spots, however, are neither permanent 

 nor unchangeable. When observed from day to day, or even from 

 hour to hour, their form is seen to change; they expand or con- 

 tract, and finally disappear ; on other parts of the solar surface 

 new spots spring into existence where none could be discovered 

 before. When they disappear, the darker part in the middle of 

 the spot contracts to a point and vanishes sooner than the edges. 

 Sometimes they break up into two or more parts that show all 

 the signs of mobility characteristic of a liquid, and the extraor- 

 dinary commotion which it seems only possible for gaseous mat- 

 ter to possess. The magnitude of their motion is very great. 

 An arc of 1 second, as seen from our globe, corresponds to 465 

 English miles on the sun's disc ; a circle of this diameter, which 

 measures nearly 220,000 English square miles, is the smallest 

 area that can be seen on the solar surface. Spots, however, more 

 than 45,000 English miles in diameter, and, if we may trust some 

 statements, of even greater dimensions, have been observed. For 

 such a spot to disappear in the course of six weeks (and they 

 rarely last longer), the edges, whilst approaching each other, must 

 move through a space of more than 1000 miles per diem. 



" That portion of the solar disc which is free from spots is by 

 no means uniformly bright. Over it are scattered small dark spots 

 or pores, which are found by careful observation to be in a state 

 of continual change. The slow sinking of some chemical preci- 

 pitates in a transparent liquid, when viewed from the upper sur- 

 face and in a direction perpendicular thereto, resembles more 

 accurately than any other phenomenon the changes which the 

 pores undergo. The similarity is so striking, in fact, that one 

 can scarcely resist the idea that the appearances above described 

 are owing to a luminous medium moving about in a non-lumi- 

 nous atmosphere, either like the clouds in our air, or in wide- 

 spread planes and flame-like columns, or in rays like the aurora 

 borealis. 



" Near large spots, or extensive groups of them, large spaces are 



