I30 



NATURE 



[December 6, 1900 



Both of these pulses have corresponding maxima in 

 the curves for the rainfalls of Leh, Murree and Newera 

 Eliya, the dates of these in all three cases being 1878 

 and 1882. 



The delay of about a year in the effect of the Mauritius 

 pulse being felt in Ceylon and India is exactly what 

 would be expected if the rain at sun-spot minimum 

 comes from the south, as has been surmised. 



The fact that the pulses at Mauritius, Ceylon and 

 India in 1882 occur simultaneously is very strong evi- 

 dence in favour of an origin in the equatorial region 

 itself for the Indian rain at sun-spot maximum. The 

 pulse at maximum in the Indian south-west monsoon may 

 depend to a large extent upon the action of the excess of 

 solar heat on the equatorial waters to the south of India, 

 and not on an abnormal effect on the south-east trade. 



We have found that there was a defect of the usual 

 rainfall at Mauritius in 1892-3, and yet the rain supply 

 in India was in excess. 



from time to time in the Southern Ocean. In his 

 "Annual Summary" for 1896 he wrote as follows : — 



" It has apparently been established in the discussion 

 that the variations of the rainfall in India during the past 

 six years are parallel with, and in part, at least, due to 

 variations in the gradients, and the strength of the winds 

 in the south-east trade regions of the Indian Ocean. 

 The discussion has indicated that there are variations 

 from year to year in the strength of the atmospheric 

 circulation obtaining over the large area of Southern 

 Asia and the Indian Ocean, and that these variations are 

 an important and large factor in determining the periodic 

 variations in the rainfall of the whole area dependent on 

 that circulation, and more especially in India. It has 

 also been indicated that these variations which accom- 

 pany, and are probably the result in part of abnormal 

 temperature (and hence pressure) conditions in the 

 Indian Ocean and Indian monsoon area may be in part 

 due to conditions in the Antarctic Ocean, which also 



MEAN -PULSE MEAN +PULSE MEAN -PULSE MEAN+PULSE (Accelerated) 



1873, 1874, 1675. 1876, 1677 , 1876 , 1879 , I860 , 1881 , 1882 . 1885, 1864 , 1 885, 1886, 1667 , 1866 , 1889 . 1890, 1891 , 1892 , 1895,1894. 1895, 1896, 



Fig. 1. 



Result of the Comparison of Rainfall. 

 The -f- and — Pulses. 



It seems quite certain that we are justified in asso- 

 ciating the 1878 pulse of rainfall during the south-west 

 monsoon in India with the rainfall common to Mauritius, 

 Batavia and the Cape at that date ; that in all cases the 

 rain has been associated with some special condition con- 

 nected with the south-east trade in the Indian Ocean. 



The rainfall of Cordoba suggests that the same trade 

 wind in the Atlantic Ocean was similarly affected at the 

 same time. 



The Cause of the - Pulse. 



Mr. Eliot long ago conjectured that the rainfall of 

 India was profoundly modified by events taking place 



NO. 1623, VOL. (ii\ 



determine the comparative prevalence or absence of ice- 

 bergs in the northern portions of the Antarctic Ocean." 



We have begun an investigation into the pressure 

 changes which have been recorded in this region, but it 

 will be some time before it is finished. The idea under- 

 lying the inquiry is that the reduced solar temperature 

 may modify the pressure so that the high pressure 

 belts south of Mauritius may be broken up and thus 

 allow cyclonic winds from a higher latitude to increase 

 the summer rains as they certainly were increased at the 

 normal minima of 1877 and 1888. 



It has been shown that the — pulse is felt in India 

 about a year later than it commences action in the 

 southern oceans ; while in some cases' the -I- pulse is 

 felt almost simultaneously in India and at the southern 

 stations. 



