.'>5H Scientific Proceedings, Royal Dublin Societ/j. 



system, rising abruptly from the sea and culminating in Lugnaquilla at 

 a height of 3039 feet. In the extreme south-west corner of the area due 

 west of Carlow the land rises rather abruptly again to nearly 1000 feet. On 

 the north of Dublin the country rises very gradually in a uortli-westerly 

 direction, passing from 100 to 250 to 500 feet and higher, as we approach 

 the north-west corner of the area. Wicklow Head, Bray Head, and the 

 Hill of Howth form conspicuous elevations along the coast. The area 

 includes all county Dublin, and nearly all of counties Wicklow, Kildai-e, 

 and Meath, with parts of counties Wexford, Carlow, and Louth. 



Mean Annual Uainfall. 



In considering the space-distribution of rainfall, the time-unit selected is 

 usually the year ; and this was adopted in the present paper for the sake of 

 uniformity with other work on local variation, and in consideration of 

 the data available. The year has the advantage as a time-unit in such 

 work, in that it forms a complete seasonal cycle as regards insolation or solar 

 influence, which is the most fundamental factor in meteorology. It must 

 be recognized, however, that for many purposes the annual rainfall is 

 not as important as that of shorter periods, and that in comparing the 

 rainfall of two stations, it is possible to get tlie same annual precipitation for 

 the two ; yet the fi'equency, intensity, and average monthly distribution may 

 be quite different in the two cases. Both the agriculturalist and civil engineer 

 are more concerned with the rainfall over short critical periods than with the 

 total for the year. There seems little probability of correlation between 

 annual rainfall and such agricultural statistics as crop-yield ; yet it is con- 

 ceivable that such a correlation might be established with rainfall over 

 certain critical months of the year. The engineer, in questions of water- 

 power, town-supply, and drainage is concerned chiefly with fluctuation in 

 rainfall, and especially with critical maxima and minima within limited 

 periods. It must thus be admitted that any complete discussion of distri- 

 bution should take account, not only of the annual, but also of the monthly 

 or seasonal rainfall. 



The term " Mean Annual Rainfall " is open to different interpretations 

 and needs some consideration. Apart from the looseness of its use in 

 popular language, the word " mean " is used in different senses in scientific 

 work. In dealing with a number of different values of the same quantity 

 such as occur in experimental results or statistics, the mean value may be 

 determined in different ways. When the quantity by its nature has neces- 

 sarily an exact value, and the differences observed are due to " errors of 

 observation," the mean of the observed values is intended to give the " true 



