Edwin. — On the Principle of New Zealand Weather Forecast. 43 



from the country. These are the special principles of New Zealand Weather 

 Forecast ; and the diagrams ( PL 11. j accompanying this paper of the pressm-e 

 within the New Zealand area on 7th, 8th, and 9th July, 1879, are examples 

 drawn in accordance with its rules, but they must not be considered specimens 

 of actual forecast^a subject which will be treated of in another paper. 



In these diagrams the wind deflections are ehminated, and they are, in 

 fact, diagrams of the results already arrived at. 



In support of the system now in use it may be urged that if according 

 to accepted principles each cyclone, or area of low pressure, is a complete 

 circle, then it follows that from whatever point pressure commenced to 

 diminish it must return to that point again as the low area passes away, 

 unless it be assumed that, instead of pursuing a dh-ect route, the cyclone 

 has moved in a more or less erratic course, but by this principle of 

 multiple areas we can readily perceive that it is possible that gale may 

 succeed gale in rapid succession, and for a considerable period, each depres- 

 sion following its predecessor, and the whole system moving in a more or 

 less curved but well-defined route ; and it obviates what has always appeared 

 to me to be an impossibility, viz. : the retrograde movement of any low area. 



Secondly: the facility with which the approach of a "backing" wind 

 can be foretold; this "backing" being in reality the advancing curves of 

 a new depression, whose approach will cause pressure to dimmish before it 

 has reached the point from which it at first commenced to fall. The proxi- 

 mity of such an area is shown by the extent of the area over which the 

 barometer is shown to be rising, and, together with the further area over 

 which the isobaric contours show that pressure is likely to increase the sea- 

 movement, has also to be taken into consideration. 



Thirdly : the advantages afforded by it for reliable forecast of sea-move- 

 ment, a point of information which is of considerable value to bar-harbours 

 and roadsteads. 



This principle of contour lines and multiple areas enables an explanation 

 to be offered as to how such complex movements, as a decrease at the ex- 

 tremes and an increase in the central portions of the country, can take 

 place; and, also, why several successive rapid movements may take place 

 in the south without being nearly so remarkably produced in the north ; 

 and it also affords a means of determining the positions of the depressions, 

 although they may lie at a considerable distance to seaward. 



It also shows the existence of what may be termed double-centred areas 

 of low pressure ; in these the barometer falls rapidly, the wind veering by 

 north and west and blowing a heavy gale, a recovery then sets in, the wind 

 changes southward of west, and the barometer rises rapidly for about half 

 ^n inch, and immediately that it reaches its highest point it commences to 



