INTEODUCTION. xxxix 



As compared -with. England, the most salient general features of 

 Ireland are its large extent of peat-bog and lake-surface, its vast 

 •central limestone plain of low elevation, and the absence of chalk. 

 Further details as to the more local physical features will be given 

 in section VIII. of this Introduction dealing with the Irish 

 Botanical Districts. 



II. Climate. 



Owing to its insular position the climate of Ireland, especially 

 in the west, is more moist and equable than in the rest of Europe 

 Tinder the same degrees of latitude. From observations made at 

 isixteen stations in the year 1851, it appears that the mean annual 

 temperature of Ireland is a little over 50° Fahrenheit, which is the 

 same as the average in South Britain. But it is not the mean 

 temperature of any country so much as the extreme ranges which 

 affect in summer the ripening of fruit and crops, and in winter the 

 preservation of tender plants. There is a striking difference between 

 Ireland and Great Britain in this respect ; for the mean tempera- 

 ture of the summer months being 2° Fahrenheit lower in Ireland, 

 the ripening of com and fruit is later and more imcertaiu than in 

 England, and wheat, which is cultivated successfully throughout 

 nearly all England, is in many parts of Ireland a precarious crop, 

 •especially in the western counties. The Spanish chestnut, the 

 waluut, and the fig, seldom fully ripen their fruit. On the other 

 hand, the winter temperature being about 2° higher in Ireland, 

 some plants thrive and flourish with us which would be kQled by 

 the cold, dry frost of an English winter. Even in the county of 

 Dublin, some species which are natives of the western coast of 

 Ireland — for example, Simethis hicolor and Urica Mackaii — sufEer 

 much from frost when planted out in the open air in the Botanic 

 Garden at Glasnevin. 



The temperature of Ireland decreases from south to north at 

 the rate of about three-quarters of a degree for each degree of 

 latitude. A sensible decrease is also experienced from west to east, 

 especially in winter. This greater mildness and equabOity of the 

 climate in the west as compared with the east of Ireland, and in 

 Ireland generally, when compared with the rest of Europe under 

 the same degrees of latitude, was formerly attributed to the moder- 

 ating influence of the great warm ocean current, the Gulf Stream. 

 But it is now held by the best authorities that this current does not 

 ■extend eastward from the American coast beyond 50° west longitude, 



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