EXCURSION XIIT. 167 



Rosa by water: floods, tides, and storms may have produced 

 the former the whole island must have been raised 25 feet 

 at least to lay dry the " Ferry-point " Glen. This name, it 

 is true, may have reference to a lake ; and certainly the 

 present mound is very like the remains of an old lake barrier. 



The .burns of Arran ai'e all beautiful and picturesque, but 

 in different styles, according to the nature of the rocks across 

 which their courses lie. Those of the granite and slate tract 

 are perhaps not the most beautiful ; but this Rosa burn, 

 especially in its lower course, and after it enters Glen Shant, 

 surpasses in romantic beauty most of the bright streams of 

 this lovely island. 



76. Over the wood which covers the eastern end of the 

 ridge on the south side of Glen Rosa a curious meteorological 

 phenomenon is often witnessed. When the wind is at S.W. 

 or W., with a damp and warm atmosphere, a column of 

 vapour is seen ascending from near the centre of the wood, 

 remaining in a nearly steadfast position for some time, and 

 then suddenly vanishing; to be again formed, and again as 

 suddenly disappear ; and this is repeated through a period 

 sometimes of several hours. For some time we supposed 

 that it was the smoke from a gipsy fire : but the explanation, 

 no doubt, is, that currents of wind descending Glen Shirag on 

 one side, and Glen Rosa on the other, produce, by the rapid 

 out-draught into Glen Shant, a partial vacuum and vortex of 

 light ascending air over the middle of the wood, which is 

 completely sheltered ; and by this relief of pressure a con- 

 densation of vapour takes place, just as a cloud is formed in 

 the exhausted receiver of an air-pump. 



We may mention now, when on the subject of meteorology, 

 an interesting case illustrating the relations in which the dif- 

 ferent kinds of clouds stand to one another : On a bright hot 

 day, after rain on the day previous, a pretty rapid current of 

 wind from W.S.W., with scattered masses of white fleecy 

 clouds, prevailed aloft from early morning as a land wind; 

 while a light east wind blew below as a sea breeze. The cur- 



