151 



METEOROLOGICAL PERIODICITY. 



Bt H. C. Eussell, B.A., F.R.A.S., Grovernment Astronomer. 



lliead before the Moi/al Society of N.S.W., 11 October, 1876.] 



A popular writer has recently said — " Surely in meteorology, as 

 in astronomy, the thing to hunt down is a cycle. If it is not 

 found in the temperate zones, then go to the frigid zones or the torrid 

 zone to look for it, and if found, above all things lay hold of it, 

 record it, and see what it means. If there be no cycle, then 

 despair for a time if you will, but yet plant firmly your science on 

 a physical basis and wait for results." * 



In the spirit of these remarks, I shall attempt to bring together 

 some facts bearing upon meteorological cycles ; but from the 

 difficulty of obtaining detailed observations for long periods and" 

 for many places, I am obliged to confine what I have to say 

 chiefly to Australia, though I hope to be able to show you that 

 we are bound by the same meteorological causes which rule the 

 northern hemisphere, except in so far as local peculiarities 

 modify the weather, which results from cosmical causes. 



On the general question, Is a meteorological period or cycle 

 likely to be found ? a great deal might be said both in favour of a 

 affirmative, and also of a negative answer. 



We know that the earth, year after year, revolves about the 

 sun at an unvarying distance ; that the sun changes declination, 

 going north and south over the same range, and in the same 

 time ; that with it follow summer and winter, trade winds, mon- 

 soons, ocean currents, and a host of natural phenomena in regular 

 succession. We know that the average temperature, barometric 

 pressure, and winds are practically constant quantities ; nay, if 

 our lot be cast in some favoured climes we can tell to a day when 

 the wuid will change and the fruitful rain come with it, and we 

 might say, with some appearance of truth, there is no cycle but 

 an annual one. 



But if we look a little deeper we find that our averages here, as 

 in other things, are very apt to mislead us, and that under all 

 this regularity there is much uncertainty. It is true that the sun 

 makes his annual excursions north and south, but we find the 

 trade wind going at times farther north and south — is strong or 

 weak, is surrounded by hurricanes, rain storms, and danger, in 



* Lockyer — Solar Physics. 



