274 PHYSICAL SCIENCE 



hook-like appendage to the line, which begins 

 as a fine point in the middle of the sun-spot 

 absorption, and ends above by fusing with the 

 C line, tells of an extraordinary outrush of cool 

 hydrogen coming from the centre of the sun- 

 spot area, and travelling outwards with a radial 

 velocity of about one hundred and twenty miles 

 a second. In its outward course it passes away 

 from the sun-spot area, and finally comes to rest 

 at a distance of thirty to forty thousand miles 

 from its point of origin. Its absorption then of 

 course coincides in spectral position with the 

 normal C line. 



Similar work, carried on in several observa- 

 tories, has thrown much light on the movements 

 of the prominences, which come into view at 

 the edge of the sun's disc, and seem to be 

 connected intimately with the spots. These 

 enormous masses of glowing gas produce 

 bright line spectra, and the displacement of the 

 lines gives the movement in one plane, while 

 direct visual observation gives that in a plane 

 at right angles to the first. Thus the motion 

 of the prominences can be specified completely. 

 Their velocities are often as high as two or three 

 hundred miles a second. 



It seems unlikely that such high velocities can 

 be the result of differences of gaseous pressure 

 and the convection currents thus engendered. 

 They are more probably to be explained by 

 the local action of some explosive source of 

 energy, by which matter is projected with great 

 violence. 



The application of Doppler's principle to 



i 



