November 1, 1888. 



♦ KNOVS^LEDGE ♦ 



19 



inquire what can be the physical explanation of such struc- 

 tures 1 



The inclination to the radial and the falling back again 

 of the branches towards the solar surface, would seem 

 to show that these structures cannot be due to matter acted 

 upon b}' a repulsive force similar to that which drives the 

 matter of a comet's tail away from the sun. And these strange 

 forms cannot be explained as due to explosive jets of gaseous 

 matter thrown upward into the coronal region and falling in 

 free space under the influence of gravity alone to the solar sur- 

 face. Such jets would, in whatever way they were foreshortened 

 by projection, always appear eithei' as straight lines or conic 

 sections aliout the sun's centre. For a ti~ajectoi'y curve will 

 be projected into a straight line if seen from a point in its 

 plane, or into another conic section if seen from a point 

 outside the plane of the trajectory. The hooks of the great 



Fig. 2.— Large Scale 

 Drawing of Structure in IS 

 Corona. 



Fig. 3. — Outline on 

 SIMILAR Scale op Struc- 

 ture in 1871 Corona. 



jet in the 1871 corona are certainly not conic sections, 

 neither are the bra,nches of Professor Pickering's ray. If 

 in the latter case the three branches belonged to three jets 

 shot up in identically the same path, or all shot up in the 

 .same plane one behind the other, so that .as seen in pro- 

 jection the upward branches of the trajectories overlapped, 

 the downward branches could not curve away in opposite 

 directions under the influence of solar gravity. Possibly 

 the left-hand hook may belong to the great .structure which 

 rises from a part of the limb nearer the South Pole, and the 

 right-hand hook may be the summit of a structui-e curving 

 from the other side of the .synclinal group ; but if so, its 

 lower part is fainter and lost to view, (^ne is only driven 

 to such a supposition by the ditllculty of conceiving of an 

 explosion at such an altitude producing branches. C'le;vrly we 

 must give up the trajectory theory as being insufficient 



alone to account for all that is observed. Pos.sibly a theory 

 which supposes jets to be thrown up in true trajectories and 

 afterwards drifted by a resisting medium may be sufficient 

 to account for the observed forms, but there are many 

 mysteries which still have to be cleared up. Why, for 

 example, in the case of Professor Pickering's great ray, 

 .should the falling branches be drifted, while the upward 

 branch remains absolutely straight. 



It may be useful here to point out some of the po.ssible 

 changes in the form of a structure which may be due to 

 perspective. A straight ray whose direction passes through 

 the sun's centre cannot be projected so as not to be radial 

 to the sun's limb ; but a straight ray which appears radial 

 as seen in projection, may possibly be inclined to or from 

 the observer, and may be very far from normal to the sun's 

 .surface at the part of the photosphere from which it springs. 

 A ray which does not appear radial as seen in projection 

 cannot be normal to the solar surface. Tliere is ample 

 evidence in the coronal photographs of the existence of such 

 oblique rays. It Ls difficult to conceive how explosions from 

 a gaseous body like the sun can give rise to oblique rays, 

 but their existence is beyond dispute. They are, I believe, 

 without exception brighter, and they are frequently broader 

 in their lower parts — sometimes appearing to spring from a 

 broadened base like a bulbous root, which may be due to a 

 real broadening of the ray, or to overlapping smaller I'ays at 

 its ba.se. 



Some of these oblique rays are straight, or nearly straight, 

 while others show considerable curvature, and others tend 

 over in one direction in theii' lower parts, and are again 

 ftirried slightly in a contrary direction above. The existence 

 of these curving r.ays, showing contrary flexure, is a matter 

 of consider.able importance, as they appear to indicate the 

 existence of an atmosphere with currents carrying the 

 matter of which the structures are composed with different 

 velocities at different altitudes. 



And yet anything like a gaseous atmosphere, where each 

 layer presses upon and is supported by the layer beneath it 

 (as in the case of the terrestrial atmosphere), is inconsistent 

 with other solar phenomena. For example, it is inconsistent 

 with the sm.all amount of light dispersed by the corona, 

 and it is inconsistent with the n.arrow spectral lines given 

 by the prominences which correspond with a gaseous 

 barometric pressure of less than .a hundredth of a 

 terrestrial atmosphere ; with solar gravity in the pro- 

 minence region twenty-seven and a half times grejiter 

 than terrestrial gi-avltyat the earth's surface, it follows that, 

 if the .solar atmosphere were in static equilibrium, the 

 pressure would (assuming uniform temperature) be doubled 

 at a level an eighth of a mile lower ; at a depth a mile and a 

 (juarter lower down the pressure would be multiplied by 2 

 to the power of ten, that is, it would be more than a 

 thous;ind times as great as at the upper level ; at a dejjth of 

 five miles the pressure would at this rate be more than a 

 million million times as great. No reasonable assumptions 

 with respect to rapid increase of temperature would counter- 

 balance such an overwhelming nite of increase of pressure. 

 The gaseous molecules and dust particles which spectro- 

 scopic and polaiiscopic observations show are present in the 

 corona, must either be moving freely in very long tnijec- 

 torif^s, or they must be maintained in jwsition Viy some 

 force counterbalancing gravity, similar, for example, to the 

 force which repels the matter of a comet's tail. 



Be that as it may, there is certainly resisting matter in 

 the coronal region which retards the upward flight of the 

 matter of solar prominences. This was firet definitely 

 shown to to the case by Mr. Pi-octor, and w.ns an important 

 step in the solai- theory. In a paper published in the 

 Monthhi Notices of the Royal Astronomical Society, for 



