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NATURE 



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matter comparatively hot. We have abundant evidence of the 

 existence of such up-rushes in the sun. Astronomers have lieen 

 long famihar with the existence of twfo solar phenomena which 

 occur together — spots and faaiha. Just as a spot represents 

 something which is blacker, and therefore colder, than the 

 ordinary solar surface, so a facula (torch) represents something 

 which is brighter, and therefore hotter, than the surrounding 

 regions. As I have said, faculre and sun-spots accompany each 

 other, and we have evidence from various, quarters that the 

 former are not merely high up in the solar atmosphere, but that 

 tliey frequently represent matter in the very act of ascending, 

 just as a sun-spot frequently represents matter in the very act of 

 falling down. 



If we turn now to those regions of the sun's disk in v\hich 

 there are no spots we do not find a uniformly luminous appear- 

 ance. We find rather a fine mottled or granular surface c insi-^t- 

 ing of certain bright patches and of others comparatively dark. 

 The black patches may perliaps be regarded as very minute sun- 

 spots, and the bright patches as faculas on a small scale. 

 Probably, too, the bright are up rushes of comparatively hot, and 

 tJie dark down-rushes of comparatively cold matter. 



Thus we may imagine that the difference between a spotted 

 and an unspotted portion of the solar surface does not consi t so 

 much in a difference in the kind of things there present as in 

 their size. In the unspotted portion we have down-rushes and 

 up-rushes side by side but on a small scale, while in the spotted 

 region we have also down-rushes and up-rushes, but on a large 

 scale. 



It thus appears that a prominent characteristic of the solar 

 surface is the presence side by side of gigantic up- and down- 

 currents, the up-rashes consisting of very hot and very bright 

 matter carried upwards from the heart of the sun, while the 

 down-rushes consist of comparatively cold matter carried down- 

 wards from above. 



We may add that this system of cuiTents appears to be in all 

 respects most powerful during periods of maximum sun-spots, at 

 which times the velocities of solar matter are absolutely enormous. 

 By a spectroscopic method we can estimate these velocities, and 

 we find that on some occasions they reach the almost incredil)le 

 speed of 150 miles per second. 



As yet, however, we have only added another to the puzzles 

 of solar physics. We began by expressing our astonishment at 

 the power which the sun possesses of continuously pouring 

 out vast quantities of heat and light, and we must now add to 

 this our astonishment at the almost incredible velocity of its 

 surface currents. We are thus presented with a couple of wonders 

 instead of one ; but is it not possible that the one of these may 

 explain the other ? 



May not these gigantic currents denote the very machinery we 

 are in search of, and in virtue of which the sun becomes able to 

 carry light and heat from the interior to the surface, so as to 

 give us a continuous and powerful supply ? 



The sagacity of the late Sir John Herschel was not behind in 

 detecting the true state of the case. He suggested the proba- 

 bihty that at times of maximum sun-spots the sun-pot, as he 

 expressed it, may be in reality boiling very fiercely, and may 

 therefore be giving us more of what we all want instead of less 

 — be in fact preparing for a banquet instead of making arrange- 

 ments for a famine. 



Indeed we may be perfectly certain that the peculiar machinery 

 which enables the sun to continue shining must be something 

 which brings up with great promptitude to the surface new 

 particles of hot matter from within, while it carries down with 

 equal promptitude those that have already performed their light- 

 giving office. 



The sun is required to fire off without intermission a vast 

 number of light- and heat-shots into space. And the battalions 

 of particles that have done their work must quickly step behind 

 to reload, while their places must be taken as quickly by 

 a fresh and unexhausted levy of particles from within. Now 

 this recruiting process, which must ejiist, can surely he 

 nothing else than those violent up-and-down atmospheric cur- 

 rents which ob.ervation reveals to us on the surface of our 

 luminaiy, and we are thus entitled, as a matter of speculation, 

 to infer that our earth will probably receive peculiarly large 

 suppilies of sunlight on those occasions when there is most 

 manifest disturbance on the surface of the sun. In fact we may 

 regard the sun as a species of heat-engine. The ordinary con- 

 ception of such an engine is that of something provided with cylin- 

 ders, pistons, valves, wheels, and other mechanical appliances, the 



furnace and the boiler being kept generally out of sight ; but the 

 physical conception is something very different from this. A 

 heat-engine, according to the physicist, is a machine having two 

 temperatures : one being that of the source of heat, and the other 

 that of the refrigerator : and it produces work while heat is 

 carried from the higher to the lower temperature — from the 

 source to the refrigerator, and not only so, but the faster the heat 

 is carried the more work does it produce. 



Here the object or end is to produce work, and the means 

 employed is the carriage of heat. But if we regard the sun as 

 an engine we may with propriety reverse this relation between 

 means and end, and look upon the carriage of heat and light to 

 the surface as the end aimed at, and the powerful surface- 

 commotion as the means by which this end is accomplished. 



I am by no means satisfied that we can fully explain why the 

 currents on the sun's surface should be so very violent as obser- 

 vation proclaims them to be, but yet it is easy to see that the 

 conditions there present are such as to favour the development 

 cf convection-currents of enormous power. Let us agree for a 

 moment to study an ordinary furnace fire. We have here in 

 the first place a carriage of hot air up the chimney which ulti- 

 mately mixes with the cold air outside, while we have in the 

 second an in-rush along the floor of the room of the cold air 

 vihich feeds the fire, and which ultimately as hot air goes up the 

 chimney and mixes with the cold air above. Now here we have 

 a true convection-current, an up-msh of hot and an in-rush of 

 cold air, and the more intense this current the more quickly will 

 the fire burn. 



It is easy to see in the first place why the hot air ascends the 

 chimney. It does so because it has been expanded by heat, and 

 is therefore specifically lighter than the cold air around it. 



But why does a thing specifically lighter than the air ascend ? 

 Clearly on account of terrestrial gravitation. If there were no 

 earth it would not a'cend at all, and if the earth were less 

 massive than it is it would not ascend so fast as it now does. 

 Clearly then the draught of our chimneys depends upon the mass 

 of the earth. 



Again, the draught will depend upon the intensity of the fire, 

 and also upon its size and that of the chimney, for it is obvious 

 that an exceedingly small fire and short chimney would not draw 

 well even though the temperature of the fire should be very 

 high. 



We thus perceive that the intensity of convection-currents 

 depends — 



1. On the temperature of the source of heat as compared to that 

 of the cold parts of the arrangement. 



2. On the force of gravity. 



3. On the scale of the arrangement. 



4. We may add that for strong currents it is necessary to have 

 some substance, such as air, that expands greatly under an 

 increase of temperature. 



And furthermore such currents are still more augmented in 

 violence by the presence of a condensable substance in the 

 atmosphere, and are thereby rendered abrupt, and, to some extent, 

 incalculable, in their operations, inasmuch as a small cause may 

 produce a very great effect. 



Now we have all these elements of pow er together on the 

 sun's surface. For in the first place the intensity of the sun^b 

 heat is very great as compared with the cold of surrounding 

 space. Secondly, solar gravity is very great, being about 

 twenty-eight times greater than terrestrial gravity. Thirdly, the 

 scale of the whole arrangement is very great ; and lastly, the 

 substance there present, gas and vapour, is one which expands 

 greatly on being heated. On the sun's surface therefore all these 

 causes of convection-currents exist in great strength ; and if we 

 be.ar in mind that they must be multiplied together rather than 

 added we shall not fail to perceive how strong must be the 

 effects which they will produce. Notwithstanding all this, it 

 appears to me that we have more to learn with respect to the 

 causes which produce the extraordinary violence of solar 

 currents. 



Although the series of sun pictures made by Schwabe is the 

 first having pretensions to accuracy, yet Prof. Rudolph Wolf 

 has endeavoured to render observations of sun-spots made at 

 different times and by different observers comparable with each 

 other, and has thus formed a list exhibiting approximately the 

 relative number of sun-spots for each year. This list extends 

 back into the seventeenth century, and is in many respects of 

 much value. 



By this means Prof. Wolf has shown that the eleven-yearly 



