Lyons — The Distribution of Mean Annual Rainfall. 865 



Supplementari/ Notes ( Wickiow). 



An interrupted record was kept at Parknasilloge (Enniskerry), for the 

 years 1877-80, 1884, 1886, 1889-91, 1898. The values are uniformly higher 

 than at Fassaroe ; and on reduction by reference to the latter, the mean for 

 Enniskerry comes to about 45 inches. The isolated records for Bray, Pem- 

 broke Lodge (1907-8), Florence Terrace (1881), and San Eemo (1903), 

 indicate a mean close to 35 inches. The annual values for Ashford (1893), 

 Avondale (1908), and Shillelagh (1908), agree with the records for neigh- 

 bouring stations. 



The Isohyetal Map. 



The most effective manner of presenting the distribution of rainfall 

 is by means of isohyetal or eqnal-raiufall lines drawn on a map. The 

 stations with the numbers representing their mean annual rainfall values 

 iire marked on the map ; and by reference to these stations, lines are drawn 

 through points having probably the same mean rainfall. The principles of 

 constructing such rainfall-maps are very fully discussed by Dr. Mill in the 

 Quarterly Journal of the Eoyal Meteorological Society, vol. xxxiv.. No. 146, 

 April, 1908 ; and a very important critical discussion of the subject will be 

 found in the Monthly Weatlier Eeview, United States Department of Agri- 

 culture, Weather Bureau, April, 1902. 



The results obtained in the present investigation permitted the drawing 

 of isohyetal lines corresponding to 28, 30, 32, 35, 40, 45, 50, and 60 inches 

 of rainfall. Following a suggestion made some years ago by Symons, the 

 increase in rainfall is indicated by increased thickness of the equal-rainfall 

 Lines. The 50- and 60-inch isohyetals are based on but few records, and, 

 being to some extent doubtful, are represented by broken lines. Where the 

 number of records was insufficient, the run of the isohyetal line was decided 

 by reference to the contour and by careful consideration of the similarity of 

 the conditions with those of other neighbouring places of known rainfall. 

 This procedure unfortunately allows a great amount to individual judgment, 

 and is open to criticism. It is, however, in the absence of records, the best 

 approved method, and is found to be justified by experience. The degree of 

 probability to be associated with the isohyetal lines is moreover likely to be 

 greater over a small area than in the case of extended regions. 



The distribution as shown on the accompanying map (see Plate XXI.) 

 is remarkable for the great range in precipitation, and for the very rapid 

 gradient or sudden change in the rainfall immediately south of Dublin 

 City. There is seen to be an area of mean annual rainfall less than 28 



