SCIENCE 



NEW YORK, MAY 1, 1891. 



EECENT PROGRESS IN SOLAR PHYSICS AS BEAE^ 

 ING UPON THE CAUSE OF THE ICE AGE. 



Among the many hypotheses invoked to find an adequate 

 cause for the glacial period, that of a time of diminution of 

 the sun's emission of heat has had little consideration. 

 Although apparently naming a cause adequate to the effect, 

 it seemed too violeat an assumption, and one opposed to 

 generally accepted fact, that the supply of heat from the sun 

 could vary to any material amount. The universal concep- 

 tion of tlie solar orb and its activities was that of extreme 

 steadiness and uniformity of behavior, as being almost an 

 emblem of immutable law. Any change or abatement in the 

 sun's energy in supplying heat and light seemed as foreign 

 to a proper notion of that body as would be a deviation from 

 punctual rising and setting as laid down in the almanac. 



Hence, in the presence of the brilliant and imposing astro- 

 nomical theory of Dr. Croll, the more obvious hypothesis 

 of solar variation lapsed out of sight. Of late, however, the 

 former theory is becoming discredited by the growing clear- 

 ness of evidence that the ice age was too recent to be ac- 

 counted for thereby. The rates of recession of the Niagara 

 gorge and of the falls of the Mississippi render it difficult to 

 account for a continental glacier still existing seven thousand 

 years ago, by an eccentricity of the earth's orbit which oc- 

 curred fourteen times as far back in the past. 



During the quarter-century since Dr. Croll's theory came 

 into vogue, our knowledge of solar physics has been enor- 

 mously developed and quite revolutionized. Possibilities and 

 probabilities as to the variability of the sun's emission of 

 heat are now well known, which then were not even matters 

 of vague conjecture. Inspection of the structure and activi- 

 ties of the sun by means of the spectroscope has wholly 

 changed the former conditions for reasoning about its varia- 

 bility. 



The most conspicuous result of this spectroscopic inspection 

 is our knowledge that the sun exhibits the most violently 

 energetic activity all over its surface, far into its depths, and 

 far outside of the photosphere. It continually generates 

 and radiates into space almost inconceivable floods of light 

 and heat. This is attended by intensely violent ebullition at 

 the surface, in which vast streams of fluid matter are con- 

 stantly flung aloft tens and hundreds of thousands of miles 

 above the photosphere. The most titanic eruptions of earth, 

 such as Krakatoa, are, when compared with those hourly 

 occurring in the sun, far less than the dust-whirl of the street 

 is to the tornado that wrecks a city. 



I adduce this fact of violent ebullient activity in the sun 

 as lending a presumption of more or less inequality in that 

 activity. It gives the impression of contending forces arrayed 

 against each other, necessarily disturbing equilibrium, and 

 forbidding an equable and uniform emission of light and 

 heat. Such inequality is markedly indicated by the known 

 periodicity of the cyclonic sun-spots, and their attendant 

 cosmic magnetic disturbances. There still lack results of 

 actual observation to verify the fact of such fluctuation. The 



younger Angstrom of Sweden is understood to be now con- 

 ducting delicate observations with this intent. 



A vastly more extended area for observation and classifi- 

 cation of facts relating to solar physics has been opened in 

 the new department of stellar spectroscopy in which Dr. 

 Norman Lockyer is the worker best known to the public. By 

 the classification of the spectra of many hundreds of fixed 

 stars and nebulse. a series of grades of solar evolution have 

 been approximately determined, beginning with suns incipi- 

 ently gathering from diffused nebulous matter, and going 

 on through successive stages of accumulation, concentration, 

 intensifying heat, culmination, decline, and approaching 

 extinction. .\]1 these stages are determined and classified by 

 the peculiarities of their spectra. Dr. Lockyer is thus en- 

 abled to write approximately the history of a sun from its 

 earliest genesis to its extinction as a luminary. Our own 

 sun has been definitely assigned by the character of its spec- 

 trum to a class of stars of decreasing temperature, which 

 have passed the culminating point of their activity, and are 

 going on towards decline, like Procyon, Capella, and Arc- 

 turus. Aldebaran, Altair, and Alpha Cygni are examples 

 of another class approaching their culmination, and increas- 

 ing in brightness. Sirius is in a still earlier and more vapo- 

 rous stage. 



While the known violence of the sun's internal activity is 

 suggestive of frequent transient variations in the amount of 

 heat emitted, the above named long progressive changes are- 

 equally suggestive of vast secular oscillations in the course 

 of the increase and decrease observed. It seems, indeed, 

 quite impossible that those long-continued progresses of in- 

 crease and subsequent decline in the heat and light of solar 

 orbs should go on with absolute uniformity of gradation. 

 All such processes of active change in nature are character- 

 ized by fluctuation, by alternating ebb and flow; and such a 

 process as this would be the last to show an exception to the 

 rule. It involves a continual balancing of mighty contend- 

 ing forces, forever swaying the resultant thermal condition 

 up and down with varying divergence from an even grade 

 of increase or decrease. 



It is only in harmony with the universal laws of material 

 activity — and it is nearly impossible to conceive it other- 

 wise — that the heat of the sun, as it slowly diminishes 

 through the ages, should at intervals make strong sweeps 

 upwards or downwards, again recovering itself to its average 

 grade of slow decline, rather than that it should progress in 

 a uniform and imperceptible diminution. It thus seems in 

 the highest degree probable that the sun is subject to con- 

 siderable secular variations in its heat, such as might have 

 caused the glacial period, as well as the just preceding age 

 of arctic warmth. 



As observed above, the enormous violence of the sun's in- 

 ternal movements, which is actually seen to exist, seems 

 necessarily to involve fluctuation in its effects. Such oppos- 

 ing energies cannot uniformly so balance each other as to 

 produce a uniform emission of light and heat. An enormous 

 expenditure of force is going on with the progressive conden- 

 sation of the vast orb. Volumes of heat and light incon- 

 ceivably great are being every instant shot forth and dis- 



