584 MR ALEX. BUCHAN ON THE MEAN PRESSURE OF THE ATMOSPHERE 



Christiania, S. Thus, this part of the great atmospheric current flows round the 

 southern region of Norway, being N. at Bergen, thence in succession N.W. and 

 W., and on rounding the coast becomes diverted into a SW. and S. wind. The 

 extraordinary deflection of the isothermal lines in the different months, as they 

 cross Norway and Sweden, is doubtless to no inconsiderable extent occasioned by 

 the opposite prevailing winds, which arise from the obstruction presented by 

 the mountain range to the prevailing atmospheric currents of the seasons. 



The prevailing winds at Upernivik, Jacobshavn, and Godthaab, in the west of 

 Greenland, appear to point to another principle. It will be seen from the Charts 

 and the Table that the prevailing winds in the winter months at these stations 

 are N.E. and E. instead of N. and N.N.W., which, from the analogy of the winds 

 at other places, they might have been supposed to be. They are thus diverted a 

 few points from their proper course in the direction of E., or, roughly speaking, 

 into a direction which is perpendicular to the line of the coast ; in other words, 

 they follow the course of the ravines. The daily observations at the Greenland 

 stations have been published by the Danish Academy of Sciences, from which 

 the averages have been calculated. A large proportion of these winds are very 

 light, being frequently marked by the observers ; that is, they were too light 

 to be represented by the scale for wind force in use (0 to 4.) Since the west coast 

 of Greenland is bounded immediately to the east by a steep high mountain range 

 covered with snow, it is probable that the direction of these winds is still further 

 modified by the same causes which give rise to the well-known class of breezes 

 peculiar to mountain districts, of which the Vent clu Mont Blanc is an example. 

 These breezes are caused by the cooling of the air in immediate contact with 

 the high ground, which, thus acquiring greater density, flows down their slopes, 

 and thence diffuses itself over the low ground as a surface wind of inconsiderable 

 depth. The Greenland stations are in those very situations which expose them 

 to this wind. It is in favour of this supposition that these easterly winds occur 

 oftenest, and blow with greatest force in the afternoon, it being at this time of the 

 day that the difference is greatest between the temperature of the low grounds 

 and that of the snow-covered mountains ; just as sea and land breezes are 

 strongest at those hours of the day, when the difference of temperature is greatest 

 between the sea and land. 



It will be observed that at St Helena the mean direction of the wind varies little 

 from month to month, — being almost uniformly from S.E. or S.; and it will also 

 be observed that the relative distribution of the pressure in neighbouring regions 

 varies little from month to month. The result is one mean annual direction of 

 nearly S.E. by S At Mauritius there occurs a little variation from month to 

 month. Thus, whilst in June, July, and August, the mean direction is about 

 S.E. by E , in December, January, and February, it is nearly due E. ; in other 

 words, during the summer season the wind shifts a few points from S.E. by E. in 



