Jan. 14, 18 86 J 



NATURE 



251 



RADIAN r LIGHT AND HEAT"- 



IV. {Continued) 



Radiation and Absorption — Celestial Applications 



T' HIS is perhaps the most suitable place for alUiding 

 to a method of obtaining a picture of the corona 

 on ordinary occasions, recently introduced by Dr. Hug- 

 gins, and which has already met with considerable 

 success. 



By using a suitable absorbing medium Huggins has 

 been able so greatly to diminish the proportion between 

 the terrestrial glare and the light from the corona, that a 

 photographic image of the regions around the sun exhibits 

 visible traces of an excess of action in certain places 

 which are probably those occupied by the corona. Plates 

 prepared in this manner were compared with those taken 

 of the corona in Egypt during a total eclipse, and the 

 comparison, made by several observers, appears to leave 

 little doubt that the object photographed is really the 

 corona. A development of this method would prove a 

 great boon to solar inquiry. 



Another result of the application of the spectroscope to 

 the sun has been the determination of the rates of motion 

 of the currents which take place in the solar atmosphere. 



This is done by the method of displacement already 

 mentioned, a motion of solar gas towards the eye pushing 

 its spectral lines to the more refrangible side of their 

 ordinary position, while a motion in the opposite direction 

 has a contrary effect. In Fig. 20 we have a representa- 

 tion of the deviation of the F line in a spot spectrum. 



By this means prodigious solar velocities have been 

 observed. In our earth when air moves at the rate of 

 100 miles an hour we count it a hurricane, but in the sun 

 we find gaseous matter frequently moving at the rate of 

 100 miles a second. 



It has likewise been observed that the velocities of solar 

 motions are greatest on occasions of maximum sun spots, 

 when there appears to be a general increase in the activity 

 of all things belonging to our luminary. 



It may therefore be said that all our observations com- 

 bine in proving how extensive the solar atmosphere must 

 be, and how enormous must be the velocities of its con- 

 stituents, and more especially of the hydrogen, which 

 enters largely into its composition. 



We have thus two facts connected with our luminary 

 which tin us with amazement. For we have, in the first 

 place, his continued ability to radiate powerfully without 

 cessation, or even apparent diminution, and we have, in 

 the second, the astounding velocities of his atmospheric 

 motions. 



A little reflection will, however, serve to convince us 

 that these two wonderful facts are intimately connected 

 together and serve to explain each other, and that in 

 truth the atmospheric motions are the very machinery 

 which enables the sun to continue his radiation. 



For let us inquire what is the essential condition of 

 such continued radiation. 



Clearly we must have some process by which there 

 shall be a continuous and very rapid stream of fresh 



' Continued from p. 38. 



particles sent to the surface of the sun. These are there 

 required to give out their light and heat, and then 

 promptly to retire, beinj replaced by fresh particles from 

 beneath, which again in their turn give out light and heat 

 and then rapidly retire. It is necessary that there should 

 be some powerful machinery of this kind, in virtue of 

 which fresh recruits shall continually be carried to the 

 front, while the exhausted battalions are promptly marched 

 behind into the magazine. 



Now such machinery is supplied in the vast and intense 

 solar convection currents by means of which the cold 

 matter from above is rapidly carried down, forming a sun 

 spot, while the hot matter from beneath is rapidly carried 

 upwards, forming a facul i. 



This ceaseless system of ascending and descending 

 currents gives rise, no doubt, to the mottled appearance of 

 our luminary, while in certain districts of the sun and on 

 certain occasions the system is swelled out into gigantic- 

 proportions, and we have a large sun spot, with its ac- 

 companying faculK. Nor is it difficult to understand why 

 convection currents should be so powerful in the atmo- 

 sphere of our luminary. The intensity of such currents 

 will depend upon the following conditions : — 



(i) Ln the intensity of the heat of the hot portions of 

 the arrangement as compared to that of the cold. 



(2) (_ n the intensity of gravity. 



(3) On the scale of the whole arrangement. 



(4) C n the presence of condensible substances in the 

 atmosphere. 



Now in the sun the heat of the hot particles is very 

 great, while the space around the sun may be taken to 

 represent something without heat. Again the intensity of 

 gravity at the sun's surface is very great, being about 

 twenty-eight times greater than that with which we are 

 familiar on the earth. 



In the third place, the scale of the whole arrangement 

 is very great ; and, lastly, we have without doubt the 

 presence of condensible substances in the solar atmosphere. 

 All these are powerful causes, and we must bear in 

 mind that they have not merely to be added, but rather 

 multiplied together. Can we therefore wonder that their 

 joint eflect is such as to raise the violence of solar storms 

 into something like 60 or 100 miles per second? 



These considerations may likewise, perhaps, serve to 

 throw light on the question of solar variability. We have 

 seen that sun spots have a period of eleven years, and 

 that near the minimum of this period there are occasions 

 when the sun is entirely without spots. 



Now it is sufficiently obvious, and has likewise been 

 proved experimentally, that a sun spot gives out less light 

 and heat than the ordinary solar surface. On the other 

 hand, the proportion which the spotted area bears to that 

 of the whole disc is insignificant, so that, taking these two 

 facts together, we should at first sight imagine that the 

 sun ought to give us very slightly less light and heat on 

 those ocfasions when there are most spots. 



I think, however, that this direct action is probably in- 

 appreciable, and that' sun spots are rather to be legarded 

 as .\ymptoms of a particular state of the sun, implying an 

 increased activity of solar con\ection currents. Now, in- 

 asmuch as the outpouring of solar light and heat is kept 

 up by means of these convection currents, we might 

 therefore expect that on occasions when such currents are 

 peculiarly powerful the sun should give out most light 

 and heat. To use the words of the late Sir J. Herschel, 

 the ■' J7c« pot" may on such occasions be boiling very 

 rapidly. 



On the whole, therefore, theory would lead us to infer 

 that the sun will be found most powerful in its radiation 

 on those occasions when it has most spots on its surface. 

 It will, however, be noticed that this is merely a theo- 

 retical conclusion, and has to be supported by evidence 

 which must of course be terrestrial. Have we, then, any 

 terrestrial evidence that the sun is more powerful in its 



