Maboh 1, 1899.] 



KNOWLEDGE. 



55. 



through algehraic addition, step by step, of a series of 

 plus and minus values of temperature, or rainfall, repre- 

 senting the relation of actual values to an average. Each 

 rise in such a curve, from one point to the next, indicates 

 a plus value, and each fall a minus value ; while the posi- 

 tion reached by the curvs at a given date shows the then 

 state of the weather account, so to speak ; it might be 

 compared to the balance at a bank after a number of 

 transactions in which money has been, now deposited, 

 now withdrawn. 



It is instructive, I think, to compare together a number 

 of rainfall curves of this character, made out for different 



fi'M 



+10 



places, and to consider them in relation to Biiickner's 

 cycle. 



In the diagram, accordingly, wiU be found such curves 

 for Rothesay and Culloden, in Scotland (<i and h), London 

 (c' Greenwich, c' Chiswick), Boston, in Lincolnshire (d), 

 and Geneva («). 



In all of these may be noticed long waves of variation, 



* TheTcrtioal scale, itn-ill be seen, is varied. In giving a curve for 

 Greenwich, I gladly take the opportunity of correcting an inaccuracy 

 in that formerly given. 



and the crests and hollows, the maxima and minima, may 

 be taken as convenient and natural points of division. 



Let us consider a pronounced curve like that of Boston, 

 and ask what it means. It may be roughly interpreted 

 thus :— From 1838 to 1849, a wet time, more wet years 

 than dry ; 1850 to 1874, a dry time, more dry years than 

 wet ; 1875 to 1888, a wet time, aU the nine years wet ; 

 from 1884 to 1897, another dry time, still going on 

 probably. The relation of dry to wet years in each slope 

 can be easily ascertained by counting. We have here 

 considered the year as merely wet or dry, but the degree 

 of wetness or dryness should properly be also taken into 

 account. 



The intervals between maxima and between 

 minima, in most of these curves, will be foimd 

 to approximate closely to thirty-five years. 

 Scotland, and the West of England, are 

 considered by Briickner " temporary " ex- 

 ceptions ; they sometimes conform to the 

 rule, and sometimes not. In "permanent" 

 exceptions the variation is opposite to the 

 normal. 



The detail of these curves need not be here 

 dwelt on. We might note a tendency to 

 retardation in the wave phases in lower 

 members of the series as compared with 

 higher. 



The London curves, and the prospect they 

 perhaps open up, may be shortly discussed. 

 The Greenwich curve (1841-97) shows a 

 minimum in 1864 and a maximum in 1882. 

 The indications of the dotted curve (for 

 Chiswick, 1826-69), must be reckoned inferior 

 in value to those of the Greenwich curve ; 

 but if we may take 1846 as the culminating 

 point of a long curve, we have the following 

 relations : — A dry time from 1847 to 1864 

 (eighteen years); a wet time, 1865 to 1882 

 (eighteen years) ; and the dry time from 

 1883, stiU going on. How long will this last 

 continue ? Another eighteen years from 

 1883 would bring us to the end of the century. 

 The curve should soon be turning up again ; 

 and we may fairly expect to enter soon on 

 another wet period, ending, say, about 1918. 

 The periods indicated by those curves do 

 not coincide exavthj with those of Bruckner 

 (top of diagram) ; nor should we expect them 

 to do so. The limits assigned are not rigid 

 for all places. 



The old question comes up : Has the sun- 

 spot cycle of eleven years anything to do with 

 our rainfall ? It is hard to trace any such 

 influence in these curves ; but that is not, 

 of course, sufficient to close the subject. 

 There are other points of view, and from 

 some of these, certain curious and interesting 

 relations may be perceived. 

 Thus, instead of considering how much rain falls in a 

 year, and the relation of this to the average, let us ask, 

 How many wet months occur in a year (calling a month 

 wet where its rain is over the average for that month) ' 

 There are fewer wet months than dry ; the yearly average 

 is nearer live than six. 



Now, in the sunspot cycle, we have a short period of 

 increase of spots (say four years on an average), and a 

 longer period of decrease (say seven years) ; and if we 

 compare different year-groups in the cycle in respect of 

 the number of wet months they contain, we meet with 



