1)IS(:ovi:hy 



181 



pound, indicating that it shines partially by reflected 

 light from the photosphere and partly by its own 

 inherent light. It has been generally conceded that 

 electrical action has much to do with the nature of the 

 corona, though Arrhenius and others have insisted on 

 the part which radiation-pressure plaj'S in its produc- 

 tion. Professor Perrine, in 1918, expressed the view 

 that the matter composing the corona, with the excep- 

 tion of the portion which gives a bright-line spectrum, 

 exists in the form of small solid and liquid particles, 

 and that this matter shines by its own radiance, while 

 the outer corona shines by reflected light. The greater 

 part of the motions within the corona, according to 

 Perrine, is due to mechanical causes, explosions, light- 

 pressure, and gravity, while he holds that the view 

 that the arrangement of the coronal streamers re- 

 sults from magnetic and electrical causes is not im- 

 probable. During the total solar eclipse of June 8, 

 191S, the Lick Observatory expedition, headed by Pro- 

 fessor Campbell, secured a number of photographs which 

 revealed arches of coronal matter surrounding the 

 principal prominences, indicating the probability that 

 the structure of the corona is largely dependent on the 

 forces producing the prominences. That these forces 

 are in great measure electrical and magnetic admits of 

 little doubt. 



In two papers, published in 1904, Mr. E. W. Maunder, 

 of Greenwich, brought out some remarkable facts con- 

 cerning the connection between the sun-spots and 

 terrestrial magnetism. In the first paper he showed 

 that, " of the nineteen great magnetic storms recorded 

 at Greenwich from 1875 to 1903, all the nineteen, 

 without exception, took place when there was present 

 on the sun a group of spots with projected area of over 

 1,000 millionths of the area of the whole disc ; or when 

 a group at one time very large had returned in a dimin- 

 ished form to the central meridian." In his second 

 paper, which has been said with justice to establish 

 " an entirely new conception of the solar action 

 in producing our magnetic disturbances," Maunder 

 showed that the cause of the magnetic disturbances is 

 associated with limited areas on the solar surface. The 

 magnetic action, he finds, does not radiate equally in 

 all directions, but along definite and restricted lines. 

 Thus, stream-lines from many spots may miss our 

 earth altogether, and thus a great spot need not neces- 

 sarily be accompanied by a magnetic storm. Maunder 

 found an analogy to these hypothetical stream-lines in 

 the long rays of the corona ; and while not committing 

 himself definitely, evidently inclined towards the view 

 of Arrhenius that the streams consist of droplets 

 formed by condensation in the solar atmosphere 

 negatively charged and driven away by radiation- 

 pressure. This hypothesis he spoke of as " entirely 

 consistent with the appearance of the coronal photo- 



graphs and with the conditions indicated by the mag- 

 netic storms." 



Many of the problems presented by the sun seem at 

 the present time as far from solution as ever. For 

 instance, the cause of the equatorial acceleration, 

 which has been aptly called a " legacy from chaos to 

 cosmos," is j'et unexplained; the reason of the solar 

 cycle, too, is obscure, although probably its explanation 

 is to be found in the fact of solar variability. That the 

 sun is, to a very slight extent, a variable star seems to 

 be definitely established by the researches of Abbott, 

 whose measures of the solar constant indicate a real 

 variability in radiation outside of, and independent of, 

 the terrestrial atmosphere. 



" The sun," Abbott concludes, " excepting perhaps 

 in sun-spots, is wholly gaseous or vaporous. Except in 

 sun-spots, the photosphere is too hot to contain solids 

 or liquids. The density of the gases rapidly diminishes 

 and their temperature rapidly falls from within out- 

 wards across the apparent boundary of the sun." 

 Astronomers differ as to whether or not the temperature 

 falls sufficiently to allow of condensation. Young 

 maintained throughout his career that the solar photo- 

 sphere consists of an envelope of clouds formed by the 

 condensation of the solar vapours cooled by radiation 

 into space. " The photospheric clouds are, of course, 

 suspended in the surrounding gases and uncondensed 

 vapours, just as clouds float in our own atmosphere." 

 Abbott beUeves, on the contrary, that no condensa- 

 tion can take place at a temperature which exceeds 

 6,000° Absolute Centigrade. He accepts the view of 

 the late Dr. Scheiner that the bright grains of the photo- 

 sphere are the wave-crests of gaseous currents of 

 different temperatures. 



The impression made on the terrestrial observer by 

 the sun is one of almost inconceivable magnitude 

 and power. The great volume of the sun, over a mil- 

 lion times that of the earth, its enormous mass, and the 

 mighty forces which are at work in the solar globe, 

 dwarf our planet and all our activities to insignificance. 

 Yet modern astronomy has shown that the sun is a 

 very insignificant member of the stellar system. The 

 majority of the bright stars which illuminate our night 

 skies are globes immeasurably superior to our " orb of 

 day." Betelgeux, the size of which was measured in 

 December last at Mount Wilson by interferometer 

 methods, has a diameter of 273,000,000 miles, so that 

 if we could conceive the gigantic star placed in the 

 centre of our solar system, not only Mercury and 

 Venus, but also our Earth and Mars would revolve 

 %vithin its circumference. Compared to these stellar 

 giants, our sun belongs to quite an inferior order of 

 celestial bodies ; and our earth, which seems to us the 

 end and aim of creation, turns out to be but a dwarf 

 planet revolving round a dwarf star. 



