Lyons — The Distribution of Mean Annual Rainfull. 367 



rainfall-maps of Ireland are those included by Dr. H. E. Mill in his 

 valuable paper, " On the Distribution of Mean and Extreme Annual Rainfall 

 over tlie British Isles " in 1903.^ Mill considered the mean annual rainfall 

 for the period 1870-1899, and also the rainfall for the extremely wet year 

 1872, and the exceptionally dry year 1887. 



The above maps, which were made on a small scale, gave only the more 

 general features of the distribution of rain, and did not attempt to show the 

 details of local variation. The present map was originally prepared on a 

 scale of \ inch to the mile, or approximately 1 to 250,000. Special attention 

 was given to the prepai'ation of the plate, so as to deal with the well 

 recognized difficulty of presenting at the same time the rainfall-distribution 

 and the physical features. This difficulty, which has been frequently discussed,- 

 was met in the present case by representing the rainfall-distribution by the 

 isohyetal lines and graduated tints ; the relief features being brought out 

 by showing the details of the river-systems and the heights of the more 

 important elevations, as well as by carefully selected hill-shading in black. 

 All roads were omitted to avoid complication of detail. The canals and 

 railways were marked in for reference. 



GrENERAL Results and Conclusions. 



The meteorological and physical importance of the local variation of rain 

 makes it desirable to consider the results of the present investigation with 

 special reference to the relation of rainfall to orography, and to the 

 agreement of our results with facts elsewhere established on the subject. 

 It is also necessary to review the current theoretical views on the problem of 

 rainfall and relief in seeking an explanation of the observed facts. 



The distribution observed in the present area shows a clearly defined 

 dependence on the marked configuration of the country. The highest 

 rainfall is found in the midst of the Wicklow Mountains ; the lowest in the 

 low-lying area enclosing Dublin and the valley of the Liffey near its 

 entrance to the sea. On the east and west and north, the isohyetals from 

 35 to 60 follow closely the limits of the mountain system. To the north- 

 west of Dublin the rainfall increases very gradually and is in close agreement 

 with the gradual increase in elevation. 



This connexion of rainfall with relief is seen in all parts of the world 

 where the mean annual rainfall is charted. It is found not only in the 

 general distribution over large areas, but even in the detailed charts of small 



' Minutes, Proc. Inst. Civil Engineers, vol. olv., p. 293. 



- See Monthly Weather Review, U. S. "Weather Burean, vol .\.\x., No. 4, April, 1902, 

 pp. 205-243. 



