METEOEOLOGICAL PEEIODICITY. 153 



the Sydney and South Head observations, it fails to represent 

 the rainfall results ; very much, however, may be said in its 

 favour, and it has been remarked also in the climate of Ceylon. 



Mr. Tytler, writing in Ceylon on January 30, 1873, says : — 

 ^' In the Tropics, at least here in Ceylon, where we enjoy the 

 regular changes of the monsoons, the basic period runs five or 

 six years dry and five or six years wet. These make eleven, 

 and they form the medium cycle of tJiree years, the grand cycle 

 of thirty or thirty-three years being three periods of the eleven 

 years cycle." It is evident, therefore, that in Ceylon some traces 

 of a three-year period may be found. 



Mr. Banken, in his work on the Dominion of Australia, speaks 

 of years of dry weather in Central Australia, followed by years 

 of drought. Making " years of a season, and not seasons of a 

 year," he thinks that the immense area of flat and heated plain 

 has a cumulative effect upon the weather, making season after 

 season dry, until the tension becomes too great, and a great 

 inrush of polar wind takes place, bringing abundance of rain, 

 which spreads all over the burnt-up plains. Once there, the 

 water takes several years to get away. It rises, and is again 

 precipitated, and flood after flood follows. He thinks that these 

 changes over the vast extent of flat country in a large degree 

 modifies the climate of its coast margins. 



Of this view it may be said that two well-known facts appear 

 to be overlooked. In the first place, the evaporation on those 

 interior plains during the summer months must at least amount 

 to 12 inches per month ; and overhead is steadily blowing the 

 return wind of the trades, which, as the moisture rises, sweeps 

 it away to the south-east, to be precipitated far from the interior 

 of Australia. 



As we have already seen, a period of five or six years has been 

 recognised in Ceylon ; and although I am unable to trace it in 

 our annual results, the monthly rain tables show some signs of 

 it. For instance, in April, 1845, there was a very heavy fall of 

 rain; again in 1850 and 1855, 18G1, and 1867. Another series 

 may be found in June, 1846 ; heavy rain in 1852, ] 858, and 

 18(34 ; but I do not think any weight can be attached to these. 

 They are selected cases, and by no means represent the general 

 character of the seasons. 



A period, however, of between six and seven ^^ears may be 

 traced in our dry years a long way back ; thus : 1872, 1865, 1858, 

 1852, 1845, 1838, 1832, 1826, 1820, 1814, 1808 ; and it is, per- 

 haps, worth remarking, the comet of Biela has a period of six 

 and two-thirds years, and, whether connected with the cause of 

 our seasons or not, has passed the earth in every one of these 

 years except the three last, which should be 1819, 1813, and 1807, 

 to agree with the comet's seven visits. 



