POPULAR SCIENCE 285 



a spot a rotation of 70 degrees in one day. As observed at 

 Stonyhurst, the sun-spot of April 16-21, 1882, revolved through 

 90 degrees in four days. From his spectroscopic observations 

 of movements in the penumbrae of spots, Mr. Evershed also 

 comes to the conclusion, that if a rotatory movement exists 

 in some spots, it is not a constant and regular feature of spots. 

 Yet some spots, by the swirling appearance of their umbra 

 and penumbra, vividly suggest a cyclonic or rotatory motion 

 in the spot. 



A spectro-heliograph is a modification of the spectroscope, 

 by which it is possible to photograph the sun's surface and 

 envelopes in monochromatic light, either for instance in the red 

 light of a hydrogen ray, or the violet light of a calcium ray. 

 Also it is possible to photograph the clouds of calcium and 

 hydrogen which exist above and around sun-spots at different 

 levels, so that a picture may be obtained of the sun in light 

 corresponding to relatively low-lying calcium, and to calcium 

 at a higher level. These spectro-heliograms, as they are called, 

 in calcium light, show that a sun-spot is covered and surrounded 

 by extensive clouds of luminous calcium vapour. Many 

 photographs of these flocculi, for so they are named, both in 

 calcium and in hydrogen light, have been made by M. Des- 

 landres and by Prof. Hale. It is found that the calcium 

 flocculi, which may be likened to the cumulous clouds in the 

 earth's atmosphere, exhibit no well-defined linear structure. 

 But the hydrogen flocculi of the higher regions of the solar 

 atmosphere exhibit a very definite structure, and appear, in 

 relation to the sun-spots they surround, as cyclonic whirls, 

 right-handed or left-handed as the case may be, which are 

 highly suggestive of lines of magnetic force. On June 3, 1908, 

 a great cloud of hydrogen, which had been photographed 

 earlier on May 29 and succeeding days at Mount Wilson as 

 hanging on the edge of a vortex structure, was suddenly sucked 

 into the spot at a velocity of about sixty miles a second. Hence 

 we derive the genesis of the idea, that sun-spots are analogous 

 to tornadoes or water-spouts in the earth's atmosphere, with 

 the trumpet-like end upwards. 



Let it be remarked, however, that these observations, and 

 the deductions from them, concern only the higher regions of 

 the gases and vapours above a sun-spot. 



When a sun-spot is observed by means of the spectroscope, 



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