POPULAR SCIENCE 



293 



Among Evershed's conclusions from his observations 

 (Kodaikanal Observatory Bulletin, No. 51, 191 5) he states that 

 detached penumbrae without nuclei or umbrse show no radial 

 motion ; that the radial motion may be continued for a con- 

 siderable distance outside the limits of the penumbra, or it 

 may stop short at these limits ; that there is usually an accelera- 

 tion of velocity from the umbra to the outer limits of the pen- 

 umbra, and then a sudden fall to zero in most cases ; and that 

 there is a tendency to diffusion of the lines in the direction of 

 displacement near the outer limits of the penumbrae in some 

 spots, indicating turbulent motion. A terrestrial analogue to 

 the radiation movement and diffusion in sun-spots would be 

 the welling-up of substances from the crater-cone of a volcano, 

 and their lateral spreading in all directions outwards. 



In Mr. Evershed's latest paper on the radial motion in sun- 

 spots (loc. cit.) he details some movements at right angles to the 

 radial movement. These movements might be interpreted 

 in various ways, but one possible interpretation is that they 

 indicate a general descent of the gases over a spot. Again, in 

 photographs taken at Stonyhurst of the calcium radiations 

 (H and K) over sun-spots, more frequently the bright calcium 

 lines are single near the centre of the umbra, but become 

 doubled with an ever-widening absorption between them as 

 they recede from the centre. An obvious interpretation of this 

 appearance is that the absorption is due to the greater quanti- 

 ties of cooler vapours, as welling up from the spot centre they 

 fall to lower levels. 



Taking into account, therefore, the Wilson effect undoubtedly 

 present in the penumbrae of round regular spots, when near the 

 sun's limbs, and the appearance of the umbra as overhanging 

 the penumbra when very near the limb, as also the spectroscopic 

 evidence, set forth above, we might picture a sun-spot some- 

 what as follows. The umbra is a mass of cooled, absorbing 

 vapours, which, mountain-like, rises above the level of the 

 photosphere into the reversing layer. Flowing from it and 

 sinking, these absorbing vapours seek a level slightly below 

 that of the photosphere, and pass into it as dark radial streams 

 giving the appearance of the penumbra. The penumbra itself 

 is in round spots in the form of a shallow saucer-like 

 cavity, the lowest portion being due to the falling-in of the 

 photospheric clouds, caused by the uprush of the mass of 



