178 



NA TURE 



[June 



1904 



in attempting to classify the pressure curves was tliat 

 some curves, after following' very closely for many 

 years the Cordoba ( — ) or Indian ( + ) type of pressure, 

 as the case may be, would revert back to the opposite 

 type for a period of years. Thus, to take the case of 

 one station alone, namely, Sydi:ey (Nova Scotia), as 

 an Instance, the pressure curve follows very closely 

 that of India from 1874-18S2, after which, up to i8go, 

 it has a very close resemblance to the Cordoba type. 

 The behaviour of this Svdney (Nova Scotia) pressure 

 curve can be compared with the Adelaide (.Australia) 

 curve in F'ig. 2, but it must be noticed that the forjiicr 

 has here been inverted. 



The accompanying map (Fig. i) shows the result 

 •of an attempt to indicate the position of a neutral 

 line to illustrate appro.ximatelv the mean lines of separ- 

 .ation of these two chief |)ressure tvpes, although it 



BOMBAY 

 INDIA 



(APR- SCFj 



BERUFJORD 

 ICELAND 



M\/^'^i 



Fig 2.— Showing the two main types of pressure 

 by Bombay ( + ), Cordoba ( - ). and the baromerr 

 Adelaide and Sydney (Nova Scotia) and be 



must be remembered that this line is liable to a prob- 

 able small oscillation about its mean position. 



So far as can at present be determined,- one line 

 commencing to the west of Alaska, separating this 

 region from Siberia, passes easterly along about the 

 60° parallel of latitude, and runs in a south-easterly 

 direction between .south-west Greenland and north- 

 east Canada. It then crosses the North .Xtlantic, 

 passing to the north of the Azores, and skirts the 

 south-western portion of Portugal. It then strikes 

 down towards the Equator, cutting north-west 

 Africa, so far as can be judged from the scant pressure 

 values available, through the middle of the Sahara. 

 It leaves .Africa near the Gold Coast, and passes into 

 the South Atlantic, where it cannot be traced further 

 owing to lack of observations in this southern ocean. 



The other boundary or neutral line passes to the 

 north-east of Greenland and north of Iceland, crosses 



NO. 1808, VOL. 70] 



the southern portion of Norway and Sweden, and 

 traverses southern European Russia. It then takes a 

 course somewhat more easterly, skirting the northern 

 part of the Caspian Sea and Turkestan, passes 

 between Tibet and Mongolia, and through China. It 

 then leaves the continent a little to the south of the 

 Yellow Sea, and passes into the North Pacific Ocean. 

 Here its path cannot be traced, but it evidently passes 

 well to the east of the Philippine Islands, and Solomon 

 Islands, takes a new south-westerly course, skirting 

 the eastern side of .Australia and passing between 

 Tasmania and New Zealand. Its track is then again 

 lost in the southern Pacific Ocean. 



Although too much weight must not at present be 

 given to the positions of these neutral lines through- 

 out their whole length, it is interesting to note thai 

 they are fairly symmetrical to one another, although 

 no attempt has been made to make them so. Both 

 lines apparently cross the equator at about antipodal 

 points, and both appear to have a similar trend in 

 northern and southern latitudes. 



The result of this survey seems to indicate clearlv 

 that there exists a general law relating to the pressure 

 changes which occur simultaneously in these two 

 extensive regions of the globe, separated and defined 

 more or less by a neutral line, this latter forming 

 a fulcrum about which see-saws of pressure from one 

 region to another take place. 



Special cases of such reverse pressure variation^ 

 have previously been noticed, chief among which 

 are those detected by Blandford, Hildebrandsson and 

 Hann. 



To illustrate these cases in relation k) the present 

 work, there have been drawn on the map (Fig. 1) 

 small circles connected by lines to show their relation 

 to the neutral line. A single glance is sufficient to 

 see that in all cases except one the see-saws occur 

 in places lying on opposite sides of the neutral line. 



These results thus agree well in the main with the 

 present distribution of the regions which liave been 

 examined. 



Quite recently Prof. Bigelow, working on the same 

 lines as those indicated in the present research, 

 published a map of the world on which he has in- 

 dicated the distribution of the pressure types accord- 

 ing as they follow the Indian (or direct type as he 

 calls it) or the Cordoba (indirect) pressure variations. 

 In most of the main features, however, his map 

 suggests a somewhat similar distribution of these 

 pressure tvpes to that given here. Thus, he finds 

 that " the region around the Indian Ocean gives direct 

 sNiichronism, South America and North .America give 

 inverse synchronism, while Europe and Siberia give 

 an indifferent type. Greenland and Iceland seem to 

 have direct type like the Indian Ocean. . . . The 

 e.istcrn hemisphere tends to direct synchronism, 

 rxirpi in I'Airope and Russi.'i where the indifferent 

 t\|ir |)ri\,iils, and the western hemisphere to the in- 

 \-erse t\"pe. '" 



It may be mentioned in conclusion that regions 

 which are the reverse of one another as regards these 

 secular pressure variations should very probably 

 experience opposite kinds of abnormal weather, while 

 tho.se over which the same type of pressure variation 

 exists should have weather of an abnormal but similar 

 nature. The intimate connection between pressure 

 and rainfall, allowing for the local conditions as 

 regards situation for the latter, suggests that the 

 variation of rainfall should be closely studied in rela- 

 tion to this barometric surge, and it is in this way 

 that progress may be looked for with regard to 

 monsoon and other seasonal forecasts. 



W'lLI.I.AM J. S. LOCKVEK 



