July 13, 1905] 



NA TURE 



249 



SOLAR .4A'D TERRESTRIAL CHANGES. 

 n^HE problem of the relations between sun-spots 



-•■ and other solar phenomena and weather has 

 engaged the attention of men of science for many 

 years past. The results of their investigations have 

 not, perhaps, been so satisfactory or conclusive as 

 were at first anticipated, but this, fortunately, has 

 not diminished the enthusiasm of those interested in 

 the solution of the problem. The ordinary public who 

 were attracted by the apparent simplicity and prob- 

 ability of the relations suggested have undoubtedly 

 been disappointed with the results. There has hence 

 been a tendencv for some time past to depreciate in- 

 vestigation in this field of science. On the other 

 hand, the experience of the recent droughts and 

 famines in India, Australia, and South .Africa has 

 directed attention strongly to the probable relation 

 between variations of solar activity and the larger 

 variations of rainfall over the earth's surface. The 

 aqueous vapour precipitated as rain over large land 

 areas such as India is produced by evaporation over 

 distant oceanic areas, and is thence carried to the 

 ■ireas of discharge bv the larger atmospheric currents. 

 These actions are the direct results of the conversion of 

 solar energy, and any large variation in the supply 

 of that energy must be accompanied with, and 

 followed bv, corresponding changes in the amount 

 of evaporation and atmospheric movement, and 

 hence, also, of amount and distribution of rain. 

 The determination of the relations thus indicated is 

 not merely of value from the scientific standpoint, but 

 has important practical bearings, as it may lead to a 

 satisfactory method of long-period weather forecasting 

 — a question which is largely engagiiig the attention of 

 meteorologists at the present time. 



Three lines of observation (and hence also of in- 

 vestigation) carried on at the present time furnish data 

 for the solution of the problem. These are the observa- 

 tions of terrestrial magnetism, of terrestrial atmo- 

 spheric meteorology, and of solar phenomena. 



.\ large number of magnetic observatories, 

 furnished with the most delicate and sensitive instru- 

 ments, provide a continuous record of the changes of 

 the earth's magnetic state by its action on magnetised 

 needles at the earth's surface. 



The work of meteorological observation has 

 made great progress during the past twenty-five 

 years. It has not only been extended and im- 

 proved, but is carried on much more systematically 

 than hitherto. Unfortunatelv its record is very im- 

 perfect, as it is probably not too much to say that 

 over at least five-sixths of the earth's surface, 

 including the greater part of the interior of 

 Asia and .'\frica, and over the larger oceanic areas 

 and the Polar regions, the amount of observation is 

 exceedinglv small and of little value for the solution 

 of the problem. There is hence a continuous record 

 of the meteorological changes of the earth's atmo- 

 sphere over barely one-sixth of its surface. There is, 

 moreover, no general collection and publication of 

 the meteorological data in such a form as to give a 

 continuous history of the larger atmospheric vari- 

 ations and changes in progress over even that sixth 

 part of the earth's surface. 



The third branch of observation, that of solar 

 phenomena, has made wonderful progress during the 

 past fifty years. Previouslv the telescopic examination 

 of the sun's surface had disclosed the eleven-vear 

 periodicity of the sun-spots. Latterly the combination 

 of the spectroscopic and telescopic observation of the 

 sun has revealed the complexity of the changes in pro- 

 gress throughout the depth of its atmosphere, and of 

 which the sun-spots are only one and a verv partial ex- 

 pression. This field of investigation is so promising 

 NO. 1863, VOL. 72] 



that solar observatories have been established in many 

 countries, and a continuous record of the solar 

 changes, so far as they are indicated by present 

 methods of observation, is now possible by combining 

 the data furnished by all the observatories. The work 

 of correlating the three classes of observation has, how- 

 ever, not yet been commenced in a systematic manner, 

 although the necessity is now fully recognised. 



It is now generally, if not universallv, admitted 

 that the sun is practically the sole source of the energy 

 which maintains the movements of the earth's atmo- 

 sphere. It is the centre of a continuous outflow of 

 radiant energy, a very small portion of which is inter- 

 cepted and appropriated by the earth, where it is 

 converted into other forms of energy. The investi- 

 gation of the rate of this flow of energy and its 

 time variations, the analysis of the total energy into 

 its elements as that of a series of oscillatory move- 

 ments of different periods and amplitudes or wave- 

 lengths, and the problem of its distribution in its 

 passage through the atmosphere and at the earth's sur- 

 face are each in little more than the initial stages. In 

 some departments of the investigation, as, for 

 example, the laws of the absorption of the solar 

 energy during its passage through the earth's atmo- 

 sphere, much work has been done, but with com- 

 paratively little result. 



The appropriation of solar energy by the earth 

 affects _ it mainly in two ways, first, as a whole, 

 determining or modifying its magnetic condition, and 

 secondly, partially, affecting the atmosphere and a 

 thin surface layer of the solid or liquid mass. .Any 

 variation in the flow of solar energv, periodic or 

 irregular, will theoretically give rise to correspond- 

 ing changes in the earth's magnetic condition and its 

 atmospheric movements. The determination of the 

 relations between the three classes of variation is 

 on the whole the most important problem in this 

 field of inquiry into the solar energy and its variations 

 and effects. 



The first part of the problem, that is, the relation 

 of the variations of solar energy (as manifested and 

 measured by the observable changes in the number 

 and extent of the sun-spots, prominences, &c.) to those 

 of the magnetic condition of the earth shown bv its 

 action on a magnetised needle suitably suspendecl, is 

 comparatively simple, as the earth appears to be 

 similarly affected as a whole and throughout its whole 

 mass. The variations are indicated as clearly and 

 satisfactorily by an observatory in India or Australia 

 as at Kew in England. There are undoubtedly local 

 variations which may require to be eliminated in 

 order to obtain the general variation. It has, how- 

 ever, been conclusively established by observations in 

 different regions that there is a general parallelism 

 between the amount and extent of the magnetic vari- 

 ation or disturbance and the number and magnitude 

 of the sun-spots and prominences. The rule is, the 

 larger the number of sun-spots the greater the amount 

 of the magnetic variation and disturbance. The re- 

 lation can, however, at the present stage only be 

 considered as statistical, as it has not been estab- 

 lished for single sun-spots. In other words, the 

 observed outburst or sudden appearance of a single 

 spot or prominence is not invariablv accompanied by 

 a terrestrial magnetic disturbance. Various reasons 

 have been given for the failure of parallelism in 

 detail. Hence all that can be inferred at the present 

 time is that definite relations (of a statistical kind) 

 of great importance have been obtained which more 

 than justify the continuance of this branch of the 

 inquiry, and make it desirable that the work of 

 terrestrial magnetic observation and investigation, 

 and of comparison with solar phenomena, should be 

 maintained and if possible extended. 



