Schaw. — Atmospheric Motion. 377 



downward currents in the air. The upward currents arein 

 areas over which a cyclonic circulation of the air exists, which 

 in this Southern Hemisphere means a circulation with the 

 hands of the clock, and screwing upwards. The upward 

 motion of the air diminishes its pressure, and so the barometer 

 falls. The downward currents are in anticyclonic areas, 

 where in this Southern Hemisphere the circulation of the air 

 is in the direction opposite to that of the hands of the clock, 

 and screwing downwards, so increasing its pressure and show- 

 ing a higher barometer. 



The upward and downward motions of the air are per- 

 formed in very complex swirls, and it is the algebraic sum of 

 the pressures "in all the superincumbent strata of air which is 

 indicated by the barometer at any place and time. 



A belt of cyclones appears to encircle the earth, having 

 their centres ordinarily in about latitude 50° S. They average 

 about fifteen hundred miles in diameter, and travel from west 

 to east at an average speed of 250 knots in twenty-four 

 hours. These cyclones are sometimes in parts of their courses 

 shifted in latitude very considerably, and their rate of travel 

 eastward is very much interfered with by anticyclones at 

 times. Always their course and outline is very much inter- 

 fered with by land, and especially by high land lying across 

 their course, as the Southern Alps of New Zealand. 



The typical form usually taken up by the northern part of 

 a cyclonic storm as it reaches the west coast of the South 

 Island of New Zealand is shown on Plate XV. (a tracing 

 from the isobars of the 11th May, 1900). The special effects 

 of this diversion of the regular circular or elliptical form of the 

 storm is to produce an eddy near Cape Farewell Spit, with 

 northerly winds on the west coast and southerly winds on 

 the east coast of the South Island, which continue southerly 

 on the south coast of the Strait and bend round to northerly, 

 reaching Wellington from the north and bending round again 

 to pass up the south-east coast of the North Island as south- 

 west winds. Moreover, very often an anticyclone is de- 

 veloped at the same time in the great loop over the South 

 Island. Thus it happens that what I had hoped would be 

 an admirable position for the balanced wind-vane to give in- 

 formation about the up and down currents in passing storms, 

 owing to its perfectly free open exposure, very often gives no 

 indication whatever, owing to the eddy or calm between the 

 beginning and the ending of the great bend or loop which 

 encloses the South Island. Occasionally, however, when the 

 centre of the storm is unusually far north, as on the 3rd 

 October, 1899, it passes through Cook Strait, and imme- 

 diately after the lowest point is passed the balanced wind- 

 vane shows a change to a down current and the barometer 



