Edwin. — On the Principle of New Zealand Weather Forecast. 41 



exaggerate whatever was due to irregularity of the land surface ; and it was 

 only by closely watching the changes accompanying the passage of every 

 low area, and patiently endeavouring to assimilate these conditions, that 

 it became evident that the complications already alluded to were the result 

 of there frequently being more than one low area in the vicinity of the 

 country at the same time. 



One of the principal aids in arriving at this conclusion is derived from the 

 substitution of contour lines for the isobar as generally drawn ; these isobaric 

 contours are drawn in the du'ection which the wind blows to — flow in fact 

 with the wind — and the idea of adopting this plan originated in an endeavour 

 to comprehend local peculiarities, for it was evident after investigating such 

 remarkable distributions of pressure as those already referred to, that, if 

 warnings were sent of an approaching gale, they must make known the 

 Hmits within which it would be locally experienced, for if the warning 

 were based upon theoretical principles it would frequently be incorrect, and 

 the calculation of gradients under these cu'cumstances seemed rather 

 difficult. 



In drawing these isobaric contours, the mobile nature of air has to be 

 taken into consideration. We all know how susceptible it is of expansion 

 or contraction, to changes of temperature and pressure ; and this being the 

 case, it is but a step further to allow that a cyclonic wind, whose shape 

 may be assumed as circular while beyond the influence of land, may 

 become much changed in shape when it approaches a mountainous country, 

 and by reference to Piddington and other writers upon circular storms it 

 will be found that this point has already been under notice ; but a little 

 further consideration will show that this is not the only difficulty that has 

 to be dealt with, as it will soon become evident that the subject contains 

 some complex features ; for while the front or advancing curves of a cyclone, 

 encountering high land, become retarded and deflected, the centre continues 

 to press forward with undiminished speed, and consequently the isobars 

 become packed in the vicinity of the retardmg influence; but from these 

 causes the curves will have a tendency to open out at the rear of the cyclone 

 (PL I., Jig. 2, J, and as the land has a retarding effect upon the cyclone's 

 advance, so will it also act upon it after the centre has passed, causing the 

 rear curves to be extended ; but the advancing curves wUl be compressed 

 under either condition (PL l.,fig. d). 



In drawing these isobaric contours, it must be constantly remembered 

 that they must never cross each other, and that the object must be to endea- 

 vour to depict a series of concentric rings more or less bent out of their true 

 shape ; each tenth of an inch of barometric difference must be thus shown 

 flowing in the direction toward which the wind moves, and each curve in itself 



