SURGES. 



191 



Similarly in the northern hemisphere Stykkisholm and Vardo have deeper surges than 

 Greenwich and Irkoutski because they are further north. Also the surges are shortest at 

 Stykkisholm which is oceanic and longest at Irkoutski which is continental. 



The above indicates a relationship which has been found for pressure instability by other 

 methods ; namely, that unperiodic pressure changes increase from the equator to the poles 

 and from the centres of the continents to the centres of the ocean. 



Annual variation of the surges. — A glance at the curves on plate II shows that in the 

 northern hemisphere the surges are the most developed in the winter months, the curves 

 being much more disturbed during November, December and January, than during June, July 

 and August. Such a seasonal variation is not clear in the Antarctic. Judging from the curves 

 for the Gauss Station and Snow Hill one would be inclined to put the period of least 

 disturbance from June to September, i.e., in the winter, but the Hut Point curve has the least 

 variation in December, January and February. One year's observations, however, are not 

 sufficient to settle this point with such irregular curves. 



Taking the four years' observations for McMurdo Sound and dividing the year into two 

 halves at the equinoxes, we have the following result : — 



Table 111. 

 Pressure surges in McMurdo Sound (Jour years' data). 



This indicates that the surges are longer and deeper in the winter than in the summer, 

 the resulting mean daily change being approximately the same. 



We have now to consider three most interesting and important questions with regard to 

 surges : — 



(a) The extent of country aSected by any one surge. 



(b) The relative intensity of a given surge at the different places affected by it. 



(c) The origin and cause of the surges. 



We shall first consider the surges in the Ross Sea area, where the stations are so near 

 together that the surges are easily recognisable at each station, and then turn to the more 

 widely separated stations in other parts of the Antarctic. 



Comparison of surges in the Ross Sea area.— A glance at plate I will show at once that 

 the same surges affect all the stations in the Ross Sea area, but this is not sufficient ; we 

 must find some method of comparing the intensity at the three stations. This can only be 

 done by recognising the maximum and minimum of the same surge at each station, and then 

 calculating the average value of a number of surges thus recognized. One gets into diffi- 

 culties, however, as soon as one attempts to do this, because there is a certain amount of 

 latitude in deciding whether or not a maximum on a curve should be considered as being 

 the maximum of a surge. Thus there is a well-marked maximum on the Framheim curve 



