174 Proceedings of the Royal Society of Edinburgh. [Sess. 
loch which is there, thinking it might afford something of interest because 
of the numerous water-birds that visit the island during their migrations. 
The loch, which is quite small, is situated in a ravine that divides the island 
obliquely in the direction S.E. by E. and N.W. by W. From the rocky and 
precipitous nature of the ravine one might imagine the pool to be a little 
lochan high on the mountains. The extensive engine-house at the east end 
and the cement dams at both the east and west ends, however, quickly dispel 
such a pleasant illusion. The water, which is maintained at a depth of 
about 7 feet by the dams, is used for the engines that generate the electricity 
for the lighthouse and the compressed air for the fog-horn. The light from 
the lighthouse, which is situated at the highest part of the island near its 
centre, is a revolving one, showing four rays of white light every half- 
minute. The light is produced from a single pair of very large carbons, and 
is one of the most powerful around the coast of Britain, although it is said 
not to penetrate to so great a distance as the light given by oil when the 
atmosphere is thick. The fog-horn, which is worked by compressed air, 
sounds four consecutive notes, namely- — high, low, high, low. The horn is 
situated at the south end of the island, the compressed air being conducted 
to it from the engine-house through a strong iron tube. Upon one occasion 
I happened to be close to the horn when a fog suddenly descended and it 
was consequently put into action. The noise was appalling, and reverber- 
ated from rock to rock as if it would rend them asunder. 
No aquatic Phanerogams or higher Cryptogams exist either in the water 
or about the shores of the little loch, but the water is coloured yellowish- 
green by the abundance of minute Myxophycese, Bacteria, Infusoria, Ento- 
mostraca, and by the waste water from the adjacent engine-house. The 
water is so discoloured that the bottom can only be seen at a depth of a 
few inches, and the engineer informed me that the discoloration is main- 
tained throughout the year. It must not be imagined, however, that the 
cliffs about the loch are bare of vegetation, for, besides grassy slopes and 
banks, the rocks and crannies are clothed with a variety of plants such as 
are common to the maritime cliffs of the adjacent mainland. 
Being disappointed with the loch, I gave some attention to the terrestrial 
flora, and the following is a compendium describing what I saw in August : — 
There is no peat on the island, and plants usually associated with that 
substance are consequently absent ; neither are there any trees, and, beyond 
two or three small gardens and an enclosure for grass, cultivation is nil. 
The dominant vegetation, as might be expected, is that of the maritime cliff 
type, the species of which are frequently dwarfed by the poverty of the soil 
and by the desiccating action of the wind. In some places a fine close sward 
