CONNEXION OF CYCLONES AND RAINFALL WITH SUN-SPOTS. 469 



Baron Grant, in his 'History of Mauritius' (p. 194), regrets the clestruction 

 of the woods near Port Louis, because, he says, the town was thereby " ox- 

 posed to the violence of the winds, as well as to the heat of the sun ; " and 

 in a footnote it is remarked, " these inconveniences, however, are fully 

 counterbalanced, if it be true that tlie cessation of hurricanes since 1789 has 

 been caused by the great diminution of the woods." 



As the 'History' was published in or soon after 1801, it would appear 

 that during the twelve years (1789 to 1801) no hurricane occurred in the 

 island. 



Now since, according to the Tables of sun-spot frequency, the years 1788 

 "and 1804 were maxima years, and the intervening minimum occurred in 

 1798, the theory would lead us to expect a comparative cessation of hur- 

 ricanes during the period mentioned. 



If time permitted I would adduce similar evidence respecting the hurri- 

 canes of Bourbon (Reunion) and other parts of the world. 



The hurricanes of the Indian Ocean are well known to be attended with 

 torrential rains. So much is this the case, that the popular belief at Mau- 

 ritius is that cyclones are the cause of our rains. Heavy rains over exten- 

 sive areas are certainly concomitant with cyclones in the Indian Ocean. It 

 was therefore determined to examine whether there was also a rainfall perio- 

 dicity. As far as the Mauritius observations went, the case was clear ; 

 but it was desii-able to extend the investigation to other localities. The 

 Queensland and South- Australian observations gave similar results; and as 

 Adelaide is far beyond the limits of tropical cyclones, it was surmised that 

 there might be a rainfall periodicity generally. The Cape of Good Hope 

 observations were afterwards found to support this view. The rainfalls of 

 England and the Continent of Europe were next examined, and also found 

 to be in accordance with the hypothesis. 



It would occupy much more time than I can at present spare to enter 

 fully into this question of rainfall periodicity. With the help of researches 

 on the same subject by Mr. Lockyer, Mr. Symons, and Dr. Jclinek, of Yienna, 

 I have now examined ninety-three tables of the rainfall for various parts 

 of the world; and I find that, with few exceptions, more rain has fallen 

 in the maxima than in the minima sun-spot years. I beg to append a Table 

 showing the general results for the different quarters of the globe. It will be 

 seen that, as far as the investigation has gone, Europe, Africa, America, and 

 Australia give very favourable results. Asia is represented by only three 

 stations, one of which is Jerusalem, where the excess of rain in one minimum 

 period exceeds the excess in the maxima joeriods for two stations in India. 

 France is the only European country (the rainfall of which has been examined) 

 that gives an unfavourable return ; but it must be remarked that we have 

 as yet got only five stations in that country, most of which are inland, and 

 that they may not fairly represent the whole country. 



By taking tlie longest possible series of observations for several stations 

 spread over the globe, a periodicity comes out ; and there is, I think, very 

 strong evidence that rainfall is subject to a secular variation, corresponding 

 with the sun-spot variation. 



Having given the facts, as far as I have been enabled to do so, I abstain 

 from making any theoretical remarks, beyond saying that if cyclone and rain- 

 fall periodicities be fully established, a similar (direct) temperature periodi- 

 city should also exist, and that sudden variations of solar heat and radiation 

 may, by disturbing terrestrial magnetism, be the cause of an increase of 

 aurora) and magnetic storms when sun-spots arc most mimcrous. 



