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TEE POPULAR SCIENCE MONTELY.— SUPPLEMENT. 



acting upon the various forms of water which ex- 

 ist in those envelopes, it is the fruitful parent of 

 rain, and cloud, and mist. Nor does it stop here. 

 It affects, in a more mysterious way, the electri- 

 city in the atmosphere, and the magnetism of the 

 globe itself. 



If the energy radiated from the sun were con- 

 stant, we should expect that the terrestrial con- 

 ditions which depend on the amount of solar 

 energy received at any one place would be con- 

 stant too. The daily change due to the earth's 

 rotation, the yearly change brought about by the 

 earth's revolution, would be there ; but there 

 change would stop. The fire, as well as the air, 

 earth, and water, would be constant quantities. 

 But, suppose the fire to be variable ; in other 

 words, suppose the solar energy to change in 

 amount from year to year. To the daily and annual 

 changes of our terrestrial phenomena would then 

 be added another change — a change absolutely ir- 

 regular and unpredictable if the variation in the 

 amount of the solar energy were subject to no 

 law ; but a change as regular as the daily and the 

 yearly one, if the variation in the amount of so- 

 lar energy were subject to a law. The period of 

 the additional terrestrial change would agree with 

 the period of the solar' change, whatever that 

 might be ; and to the daily and yearly response 

 of the earth to the solar energy, there would be 

 superadded an additional change, depending upon 

 and coincident in the main with the period of the 

 solar change. We have said coincident in the 

 main, because it is easy to imagine, in the case of 

 meteorological phenomena dependent upon a long 

 train of intermediate influences between the im- 

 pact of the solar energy and the final result, that 

 time would be taken for their development. In 

 this case, although the dependence would be 

 there, an exact coincidence would not. There 

 would be a lagging behind, and this lagging be- 

 hind would possibly not be the same at different 

 latitudes. 



We come now to the facts, accepting sun-spot 

 frequency as the index of solar activity. With- 

 out dwelling upon previous work, the actual enu- 

 meration of sun-spots wss undertaken in 1826 by 

 Hofrath Schwabe, of Dessau, and patiently car- 

 ried out by means of a daily scrutiny of the sun's 

 surface, nis eye-observations have been im- 

 proved upon by accurate measurements of the 

 solar-spotted area, by the late Mr. R. C. Carring- 

 ton at Redhill, and by the solar work at the Kew 

 Observatory, conducted by Dr. W. De la Rue and 

 Prof. Balfour Stewart. Similar observations are 

 now in progress, and photographs of the sun-spots 



are being taken in France, Germany, Russia, Italy? 

 and Greenwich. Dr. Rudolf Wolf has reduced 

 the materials thus obtained to a uniform stand- 

 ard, and published a list of the relative number 

 of sun-spots for each year since 1*750; the data 

 for the earlier years being, however, of less value 

 than for the later period, during which daily de- 

 lineations of the sun's surface have been going 

 on. Dr. Wolf's list exhibits eleven complete 

 cycles of sun-spots, from 1750 to 1870, giving an 

 average of, as nearly as possible, eleven years to 

 each cycle. The individual cycles vary within 

 certain limits, but the largest variations appear 

 in the last century and early in the present one, 

 before the commencement of Hofrath Schwabe's 

 continuous observations in 1826. 



.Are these cycles of solar activity coincident 

 with any well-marked cycles in the atmospheric 

 or other conditions of the earth ? The inquiries 

 into such a coincidence have been directed to 

 four classes of terrestrial phenomena. They are : 

 1. Periodical variations in terrestrial magnetism 

 and electrical activity ; 2. Periodical variations 

 in temperature ; 3. The periodicity of wind-dis- 

 turbances, hurricanes, and cyclones ; 4. Perio- 

 dicity in the rainfall. It is with the last class of 

 phenomena that we have specifically to deal in 

 this article. But it may be well to summarize the 

 results arrived at with respect to the first three. 



First, then, with regard to terrestrial magne- 

 tism and electrical activity. A freely-suspended 

 magnet, although it points in one direction, is, 

 nevertheless, within small limits, always in mo- 

 tion. Certain of these motions depend, as is 

 well known, upon the hour of the day, but the 

 magnet is also liable to irregular, abrupt fluctu- 

 tions, which cannot be connected with the diur- 

 nal oscillations. While Hofrath Schwabe was en- 

 gaged in delineating the sun-spots, Sir Edward 

 Sabine was conducting a series of observations 

 with regard to these spasmodic affections of the 

 needle. He found that such fluctuations are most 

 frequent in years of high sun-spot activity.. Van 

 Swindell had suggested, but only suggested, a 

 periodicity in the irregular movements, as far 

 back as 1785. Gauss had made further discov- 

 eries between 1834 and 1837. Arago's obser- 

 vations from 1820 to 1830 were reduced and pub- 

 lished in 1854, in such a form as to prove that I 

 a minimum period of magnetic variations had 

 occurred in 1823-'24, a year of minimum sun- 

 spots ; and that a maximum period of such vari- 

 ations had occurred in 1829, a year of maximum | 

 sun-spots. In 1S51 Dr. Lamont, of Munich, pub- 



