368 Scientific Proceedings, Royal Dublin Society. 



areas as in the present ease. It is moreover observable in districts the 

 relief features of which are not very prominent, as was shown by Hellmann^ 

 in the case of East Prussia, and by Mill- in tlie maps of Sussex and Kent. 

 Apart from the general fact that over elevated country the rainfall is greater 

 than over the plains and low-lying districts, no quantitative or definite 

 relation is discoverable between the annual rainfall and the absolute altitude, 

 and many exceptions appear to the rule— the greater the height the greater 

 the rainfall. The aspect of the hills with reference to the prevailing winds, and 

 the detailed configurations of the mountain systems, are factors as important 

 as actual height. Moreover, experience shows, and theory suggests, that the 

 rainfall will depend on the slope, as distinct from the heiglit of the mountain, 

 and on the extent of the sea or evaporating area over which the wind has 

 previously passed.^ To the lee-side the rainfall is usually found to be 

 small, as if the mountains had removed the rain from the air-currents, and 

 thus sheltered the district leewards. 



These points are prominently established in the present distribution, 

 especially when we consider the winds prevailing over the area. The ten 

 years' record, 1898-1907, at the Ordnance Survey, Phoenix Park, Dublin, 

 gives the following as the average number of days per year during which 

 the wind blows in the particular direction stated : — 



N. NE. E. SE. S. SW. W. NW. 

 18. 19. 35. 20. 27. 72. 104. 17. 



It is easily seen that the westerly and south-westerly are the most 

 prevalent winds ; and we may regard them both as rain-bearing, more 

 especially the south-west. As shown in the map, the increase in the rainfall 

 to the south-west or windward side of the mountains is rather gradual, 

 whereas on the north-east or lee side there is a very rapid decrease towards 

 the city. This effect is elsewhere recognized in the influence of mountains 

 on precipitation. On the east coast of Wicklow the rainfall gradient is more 

 gradual than that shown to the north-east. The easterly and south-easterly 

 winds are responsible for a portion of the annual rainfall near the coast ; and 

 this effect very probably masks the sheltering action of the hills on the 

 winds of westerly type. The particularly low rainfall of Kingstown and the 

 elevated district near Dalkey and Killiuey is probably due to the protecting 

 influence of the Dublin Hills. It would be particularly interesting to study 

 liow the rainfall of these districts fares in the course of easterly and south- 

 easterly winds. 



The rapid variation in the rainfall immediately south of Dublin, combined 



' loc. cit., page 355. - loc. cit., page 365. 



^ Dr. A. Buchan, Minutes, Proc. Inst. Ciyil Engineers, vol. dx., p. 135. 



