578 



H. MOHN. METEOROLOGY. 



[NORW. POL. EXP. 



direction will more or less distinctly indicate the bearing of the centre when 

 it was nearest to the ship; while a line perpendicular to this bearing will 

 indicate the direction of the track of the centre. If the wind changes to the oppo- 

 site direction, and the sector covers about 180 degrees, then the centre passes the 

 ship, and the amount of pressure in the centre at the same time may be ascer- 

 tained. A useful test is to make the diverging gradient lines, reckoned from the 

 ship's position, successively shorter with falling barometer and longer with rising 

 barometer; but this cannot be done in all cases, as the lowest pressure appar- 

 ently sometimes occurs before or after the nearest approximation of the centre. 



The change of the direction of the wind, or of the corresponding gradient 

 lines, may also be irregular, sometimes going opposite to the usual direction 

 of change. 



Irregularities such as these, where the motion changes its direction, even 

 to the direct opposite, and also its rate, and the pressure changes at the very 

 centre of a depression, are phenomena with which we are well acquainted 

 from our own cyclonic systems. 



From the curves registered by the barograph, I took out the cases in 

 which a depression amounting to upwards of 10 millimetres was observed. 

 For each such case I drew, by means of the observed directions of the wind, 

 the gradient lines and the resulting bearing of the centre of the depression 

 when nearest to the ship, and the direction of the track of the depression. 1 

 In some cases it was impossible to discover any regular motion of the depres- 

 sion. These cases are omitted in the following Table. 



The Table gives, for each case, 



(1) The number; (2) the year, month and days; (3) the duration of the 

 passage in days, approximately; (4) the direction of the motion of the depres- 

 sion, the point of the compass (true direction) towards which it moves; (5) 

 the bearing of the nearest approach of the centre to the ship ; (6) the lowest 

 pressure observed --at sea level; and (7) the highest wind-velocity observed 

 during the passage. 



The same method has been used by professor HILDEBRANDSSON in his discussion of 

 the observations from Pitlekai. (Vega-Expeditionens vetenskapliga lagttagelser, utgifna 

 af A. E. NoRDENSKiOtD. FOrsta Bandet p. 597). 



