402 



NA TURE 



\_SepL 9, 1875 



a certain force, it is easy to determine the direction in 

 which the centre of the cyclone will be found. The 

 following is Piddington's rule : — Turn the face to the 

 wind and stretch out the right arm ; the centre is in this 

 direction. The left arm must be used when a ship is in 

 the southern seas. Soon the wind increases, and the fall 

 of the barometer becomes more rapid ; the centre is getting 

 nearer, for the cyclone has an onward motion. If the 

 wind continues to increase without changing direction, 

 you are in the very path of the centre, and soon you will 

 be in the very heart of the tempest Then suddenly a 



calm ensues ; at the centre of the cyclone exists a circular 

 space where a relative calm prevails. There the sky 

 reassuming its serenity, the sailor might be led to believe 

 himself safe; but this space is soon passed, and imme- 

 diately the tempest recommences. Only the wind has 

 suddenly jumped round 180 degrees ; it blows now in the 

 direction opposite to the previous one, at right angles to 

 the trajectory of the centre of the cyclone. 



The situation which we have just supposed is a peculiar 

 case ; in general the vessel will be found to the right or 

 the left of this trajectory, whose direction, moreover, aa 



Fig. 2. — Hurricanes of the northern hemisphere (July to October). 



/ 



hni tJc nord rl( 7(i/i<! \ L\ 





\- -r- 



^ 



c 





)- y 



Zone f/cs vciils ucinaMnT''ef''"cf^('¥'ciffmcT^ 



Hurricanes of the southern hemisphere (January to April). 



attempt must be made to determine.* The alternative is 

 far from being a matter of indifference ; it is a question 

 of life or death, for the one corresponds to the favourable 

 semicircle, the other to the dangerous. The following is 

 Reid's rule, which eliminates all uncertainty : — In what- 

 ever hemisphere, if the wind changes direction succes- 

 sively by turning in the same direction as the cyclone 

 itself, the favourable semicircle is indicated ; if the wind 



* We do not dwell on this last point, which can only be solved by skil- 

 fully comparing the indications of the.barometer with those of the direction 

 and force of th« winds. 



changes by turning in the direction opposite to that of 

 the proper rotation of the cyclone, the dangerous semi- 

 circle is indicated. 



This may be accounted for by examining Fig. 3. The 

 observer, supposed to be immovable, has his face turned 

 towards the series of winds which will strike him succes- 

 sively as the cyclone passes over him. 



In the favourable semicircle (southern hemisphere), if 

 the ship behaves well in a rough sea, it is possible to 

 avoid the centre and the cyclone itself by the shortest 

 way, perpendicularly to its trajectory. The storm is 



