Edwin. — On the Principle of ISfew Zealand Weather Forecast. 45 



detected by pressure being lowered by successive steps, none of which are 

 immediately recovered. These binding-lines are an indispensable feature of 

 the work, and to explain them more readily it is necessary to refer to the 

 accompanying diagrams (PL III.j, the first of which is intended to illustrate 

 the passage of a system whose low areas travel on a route inclined about 67° 

 from the true North or E.N.E. It extends over a period of eleven days, an 

 interval which has been, for convenience-sake, extended to fully one-thu-d 

 more time than such a system would usually occupy. Each division upon 

 the Hne of route represents 24 hom-s, fm'ther subdivided into 12-hour spaces ; 

 and by moving the diagram on the line of route, making each division 

 coincide with that upon the fixed line, and marking the barometer readings 

 at Hokianga, Wellington, and Bluff upon the usual form of register, an 

 illustration will be obtained of how movements, which do not appear to 

 have much in common, may be shown to be the result of one system of 

 depressions, and that they are in reality reciprocal.* 



This diagram shows that on the Jirst day the barometers were 30-55 at 

 Hokianga, wind north-west ; 30-47 at Wellington, wind north-north-west ; 

 and 30-30 at Blutf, wind north. As we advance the diagram to the right, 

 we find, after an inverval of tivelve hours, that it has fallen nearly a tenth at 

 Hokianga, five-hundredths at Wellington, and one-tenth at the Bluff, but 

 without any material change in the wind-direction, though it will have 

 increased in force, and would under these circumstances amount to a 

 strong wind at places in the South Island. By the second day we find 

 pressm-e still diminishing, and that during the last twelve hours the 

 barometer has fallen to 30-40 at Hokianga, 30-30 at Wellington, and to 

 30-00 at Bluff. The wind has at each of these places changed more towards 

 west, backing according to meteorological law, but veering according to our 

 views, and a heavy northerly gale is now blowing at places lying southward 

 of the contour of 30-30, there being also a heavy north-west sea at Grey- 

 mouth and Hokitika. A further interval of twelve hours shows that the 

 barometer is still falling ; and on the third day it reads 30-25 at Hokianga, 

 wind west ; 30'10 at Wellington, wind north-west ; and 29-60 at Bluff 

 wind about north-west. The total fall at this station now amounts to seven- 

 tenths of an inch in three days, which would in reality have occurred within 

 one, but it has been extended for the sake of keeping the curves further 

 a]part. By continuing the movement of the diagram to the right, we find 

 that within the next twelve hours the barometer at Bluff makes a further 

 dovsmward movement to 29-65, giving a total decrease of seven-and-a-half 



[*NoTE.— For the moving diagram, a chart has been, for convenience sake, substituted, 

 on which the position of New Zealand is depicted in relation to the isobaric contours at 

 successive periods of two days' interval (PI. III.j.] — Ed. 



