Knowlks — The Maritime and Marine Lichens of Howth. 81 



Climate. — The climate of Howth is mild and equable. There is seldom 

 any frost or snow. Rain falls on a comparatively great number of days 

 throughout the year. No details have been published about the mean annual 

 temperature or the mean annual rainfall; but Sir John Moore informs nie 

 that the climate of Howth is more maritime and rather drier than that of the 

 city of Dublin. In his book on "The Climate of Ireland" (18) the mean 

 annual temperature for the city of Dublin, calculated on a mean of forty 

 years, is given as 49"5° F., and the mean annual rainfall as 28 inches. 



Owing to the steep character of the northern and eastern coasts, the 

 cliff-faces receive very little direct simlight. The morning sua soon passes 

 over to the south-west, leaving them cold and shady. This, no doubt, 

 accounts for the prevalence of alpine species on these coasts. The less steep 

 south and south-west coasts are bathed in the afternoon sunshine, and face 

 the prevalent winds, which bring moisture^ The lichens of these coasts 

 therefore grow under mucli more favourable conditions. The Broad Strand 

 is one of the sunniest and most sheltered spots on the whole coast of Ireland ; 

 and on its earth-covered cliffs several species of lichens occur that have so far 

 been found only in the Channel Islands. 



Moisture. — The main source of moisture is the rainfall, but fogs are 

 frequent in late autumn and winter ; and at these seasons the whole headland 

 is frequently enveloped in mist. During the winter months the cliff-faces 

 are often dripping wet from surface drainage, but in the summer this supply 

 is largely cut off and the vegetation is sometimes subjected to long spells of 

 drought. There are no streams of importance flowing into the sea; but here 

 and there are small rills, and several springs and small waterfalls which over- 

 flow and spread over the cliffs and the rocks on the shore. The most important 

 of these is White Water Brook ; others occur at Red Rooks and Balscadden 

 Bay. Tlie shore rocks below Earlscliffe are in some places kept constantlydamp 

 by fresh water oozing from the base of the cliffs. At these places, and where 

 fresh water flows, quite a different set of lichens and algae are to be found. 



Exposure. — No part of the coast can be described as exposed, though 

 there are places that are relatively much more exposed than others. For the 

 greater part of the year the seas are calm ; and except during storms, which 

 are of rare occurrence, the main part of the cliff-faces lies beyond the reach 

 of the spray. Nowhere on the Howth coast are the cliff-faces subjected to 

 the perpetual drenching that those on the west coast of Ireland experience ; 

 and, as a consequence, the area covered with marine lichens is much more 

 restricted. Owing to the steep and indented nature of the Howth coast-line, 

 sheltered and exposed conditions alternate with some rapidity. 



The algae give, perhaps, the best indication of the amount of exposure to 



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