34 



KNOWLEDGE & SCIENTIFIC NEWS. 



[Feb., 1905. 



bearing current for one part of a continent may under 

 similar low pressure conditions be a dry one for other 

 parts and vice versa. The lie of the land in relation to 

 the water surface must, therefore, in every case be taken 

 into account, and it is for this reason that the direction 

 of the prevailing winds becomes one of extreme impor- 

 tance. 



In the first place, let the pressure and rainfall of 

 the west coast of India and Ceylon be compared, 

 but in both of these cases the rainfall of the south-west 

 monsoon period will be dealt with alone. India, as is 

 well known, receives its greatest quantity from the 

 strong moist air current which strikes the country from 

 its south-west quarter. The strength or weakness of 

 this current means prosperity or poverty to the country. 

 A failure of these rains foretells for many districts a ter- 

 rible drought and consequently loss of crops, a famine, 

 and a great expenditure of money. Since the west 

 coast of India is most exposed to the south-west monsoon 

 wind, and is not shielded by mountains except on its 

 eastern side, this region should respond in its rainfall to 

 the pressure variations. In the Ceylon rainfall curve 

 only those months ha\ e been included during which the 

 south-west monsoon wind is blowing. 



In order that the reader may more easily compare 

 curves of pressure with those of rainfall, the former have 

 been inverted. The highest points of the pressure curves 

 therefore mean lowest pressures, and these correspond 

 with the peaks of the rainfall curves which denote years of 

 greatest rain. A glance now at the accompanying dia- 

 grams (Fig. 9) will illustrate the close resemblance 

 between these two meteorological elements. In both the 

 rainfall curves similar kinds of variations seem to exist, 

 but the rainfall of Ceylon appears to anticipate to a small 

 extent that of the western coast of India. Thus years 

 of low pressure for these regions mean, on the average, 

 years ot good monsoon rains. 



Now, not only does this connection hold for this 

 portion of India, but the same happens in the case 

 of some parts of Australia. Years of deficient 

 pressure there mean years of excess rainfall. Since 



Ig7(j0 l<!80 l89Ud IbuOo 



■ I I .... I 



PHESS'jaE"' 

 BOMB.^Y ' 



but the millions of sheep that have died through want of 

 water in the last few years indicate the importance of the 

 value of rain. 



In our own isles a similar relation of pressure and 

 rainfall holds good. Low pressure on the a\ erage means 

 a greater number of cyclones, while high pressure means 

 anti-cyclonic conditions on the average. The rainfall of 

 Great Britain on the whole is chiefly dependent on the 



PRE&iURt 

 BOMBAY. 



— Curvea lo show the relation between years o( low pressure 

 over the India area and the rainfall on the westcoa.tt ol India and 

 Ceylon dur n« the £outh-We.M Monsoon period. 



the pressure variations are nearly similar to those 

 in India, good rains should occur in the same years. 

 The annexed diagram (Fig. 10) shows the state of 

 affairs at Adelaide, Perth, and Albany, the pressure 

 curves of all these places being very similar. Excess 

 low pressure corresponds to excess rainfall. What lack 

 of rain means to this colony only those who have ex- 

 perienced a droughty season there can vividly testify; 



I670O 1880 1830 1900 O 



es to show the close relation between the pressure 

 in India and Australia, and (he rainfall in tlie latter 



winds which reach this country from the Atlantic, that 

 is south-west winds, or, in other words, on cyclones which 

 pass over the country in a direction north-eastwards. 

 Since cyclones denote low-pressure areas, the rainfall is 

 directly dependent on pressure. A perusal of the accom- 

 panying diagram (Fig. ii) shows how intimate this 

 relationship between rainfall and pressure is, for the 

 curves (the pressure curve is here inverted) are so very 

 closely similar. 



Unfortunately, the British Isles, which display pressure 

 variations intermediate between India and Cordoba, are a 

 sort of half-way house, and have therefore rather a mixed 

 type of pressure variation ; there is thus some difticulty, 

 with our present knowledge, in foretelling a year in 

 advance whether the pressure will be in excess or 

 deficient. 



Although this short-period \ariation of pressure is, 

 perhaps, the most important that is indicated in meteoro- 

 logical observations, and the reader can judge this from the 

 curves here shown, it is not the only one acting. In 

 many cases that have been examined, the most distinct 

 variations are those which extend over several years, 

 and correspond to the thirty-five year sun-spot variation 

 previously described, and to that covering about eleven 

 years. The prominence record is not sufficiently long 

 to say whether this class of solar disturbance has a 

 period of variation of thirty-five years, but the eleven- 

 year change is most pronounced. 



The thirty-five year weather cycle, or Bruckner cycle, 

 as it is called, because Briickner was the fiist to clearly 

 demonstrate its existence, has for many years been sug- 

 gested. Thus Bacon many years ago wrote: "There is 

 a toy which I have heard, and I would not have it given 

 over, but waited upon a little. They say it is observed 

 in the low countries that every five and thirty years the 

 same kind and suit of years and weathers come about 

 again ; as great frosts, great wet, great droughts, warm 

 winters, summers with little heat, and the like, and they 



