13S 



THE POPULAR SCIENCE MONTHLY.— SUPPLEMENT. 



average rain-supply brought by the forty years was 

 close on sixteen per cent, greater in Mysore than 

 the rainfall in the years of minimum sun-spots. 



To Bombay the northeast monsoon brings 

 scarcely any rain, and the returns lately published 

 omit it as being " immaterial " in twenty out of 

 sixty years. The southwest monsoon is at Bom- 

 bay the great ' factor of rainfall. According to 

 those returns, the rainfall at Bombay was more 

 or less below the average in every one of the six 

 years of minimum sun-spots during the sixty 

 years. The average rain-supply of the sixty years 

 was eighteen per cent, greater than the average 

 rainfall in the six years of minimum sun-spots. A 

 ■well-marked coincidence exists between the eleven 

 years' cycle of sun-spots and the* rainfall at Bom- 

 bay. This will be clearly shown in Table III. 



Passing from these tw T o points on the great 

 Indian Ocean lying north of the equator, to an- 

 other station in the south, we find similar results. 

 The periodicity in the rainfall of the Cape of Good 

 Hope is even more strongly disclosed in the fol- 

 lowing table than that of Madras or Bombay. 

 The Australian stations do not lie upon the Indian 

 Ocean, and are separated from it by a great con- 

 tinent. The evidence which they yield on the sub- 

 ject is meagre and irregular ; but, such as it is, it 

 scarcely bears on an inquiry which deals with the 

 water-supply collected by the great periodical 

 winds from the Indian Ocean. 



The collateral evidence with regard to a com- 

 mon periodicity between the sun-spots, wind-dis- 

 turbances, and rainfall, may therefore be ranged 

 under ten heads. These are : first, magnetic de- 

 clination; second, electrical displays (auroras); 

 third, Dr. Meldrum's list of cyclones in the Indian 

 Ocean ; fourth, M. Poey's hurricane-lists for the 

 West Indies ; fifth, the marine casualties posted 

 on Loyd's Loss-book ; sixth, the rainfall at Ma- 

 dras brought by the northeastern, and seventh, by 

 the southwestern monsoon ; eighth, the annual 

 rainfall at Madras; ninth, the annual rainfall at 

 Bombay (almost entirely brought by the south- 

 western monsoon) ; and tenth, the annual rainfall 

 at the Cape of Good Hope. We have stated the 

 facts as regards solar radiation and mean tempera- 

 ture ; but they do not, in our opinion, supply a suf- 

 ficiently firm basis for induction. The rest of the 

 evidence is exhibited in the table on the next page. 



The main point of inquiry in that table may 

 be thus stated : Is the variation in solar activity, 

 as indicated by the waxing and waning of solar 

 up-rushes, spots, and prominences, reflected in 

 terrestrial phenomena? Consequently, does a 



common cycle exist in solar and terrestrial phe- 

 nomena, in addition to and independent of the 

 two ordinary cycles, caused by the diurnal and by 

 the annual revolutions of the earth ? 



To answer this question we have examined 

 the results separately arrived at by students of 

 five classes of phenomena ; namely, the sun-spots 

 as an index of solar energy, terrestrial magnetism, 

 temperature, wind-disturbances, and rainfall. We 

 find that as regards sun-spots and terrestrial mag. 

 netism a common cycle of eleven years is now an 

 established fact; that there are indications (al- 

 though not proofs) of an eleven years' cycle in 

 solar-radiation and mean temperature ; that there 

 is ample evidence of such a cycle in wind-disturb- 

 ances ; and absolute proof of a cycle of eleven 

 years in the great factors of tropical rainfall. We 

 further find that the eleven years' cycle in the 

 separate classes of terrestrial phenomena corre- 

 spond with the eleven years' cycle of sun-spots; 

 and that, with regard to the three sets of terres- 

 trial phenomena on which we possess fullest evi- 

 dence (magnetism, wind-disturbances, and rain- 

 fall), the correspondence is most clearly estab- 

 lished. At the commencement of the paper we 

 saw that on a priori grounds, arrived at from re- 

 cent solar work, there was reason to suspect an 

 eleven years' cycle common to the phenomena of 

 the earth and the sun. We have now shown, by 

 an induction from widely-separated but converg- 

 ing series of facts, that such a cycle exists. 



This induction has a very practical interest. 

 We have seen that the eleven years' cycle in ter- 

 restrial magnetism has a direct and important in- 

 fluence on telegraphic enterprise; that the cycle 

 of wind-disturbances produces distinct results 

 upon the percentage of casualties among the ship- 

 ping of the world ; and that the cycle of tropi- 

 cal rainfall has a portentous coincidence with a 

 cycle of famine. One of the writers of this ar- 

 ticle has dealt with the subject purely as a statis- 

 tician, whose duty it was to collect and tabulate 

 all collateral evidence bearing upon the discovery 

 which he had made regarding the cyclic charac- 

 ter of the factors of the Madras rainfall. The 

 other writer has reexamined that evidence in its 

 bearings on solar physics. The conclusions at 

 which they have jointly arrived are: 1. That, not- 

 withstanding many apparent anomalies and a 

 large area of unexplained facts, the evidence suf- 

 fices to establish the existence of a common 

 cycle ; 2. That the subject merits the earnest at- 

 tention both of men of science and of those who 

 have to deal with the great present problem of 

 Indian administration. 



