Dr Hibbert on the Currents of Tide of the Pentland Frith. 57 
extremities of the Shetland Archipelago, they would be natu- 
rally opposed to each other. A gentleman has remarked to me 
that he has been for five days becalmed in a sloop between Fit- 
fiel Head and Sumburgh Head, which are distant from each 
other about three miles, without being able to pass either point ; 
one current carrying the vessel into the eastern, and the other 
into the western ocean : the sloop was often transported by the 
tide very near the shore, yet another tide always carried her off 
again. But although there is an opposition of currents from 
Sumburgh to Fair Isle, and no doubt from thence to Orkney, 
the Roust is that part of the stream lying at a small distance 
from the promontory, the force of which is probably increased 
by its proximity to the coast, and by the shallowness of the 
water. Here there is always a heavy sea, but in a storm the 
waves are said to rise mountains high. Drayton has given a 
good description of the occurrence of similar phenomena at the 
Race of Portland, not however umnixed with a tolerable pro- 
portion of poetic bathos : 
u Some coming from the east, some from the setting sun, 
The liquid mountains still together mainly run, 
Wave woundeth wave again, and billow billow gores, 
And topsy-turvy so fly tumbling to the shores.” 
Having offered these preliminary remarks on the currents of 
tide which occur among the islands of the north of Scotland, I 
shall now advert to the industrious researches that were made 
about fifty years ago, by the Reverend George Low, an acute 
naturalist of Orkney, on the coasts of the Pentland Frith, the 
explanation of which is equally connected with the view that 
they arise from lesser currents generated during the progress of 
a wave of tide round the British Isles, setting sooner on the 
west than on the east coast of the Orkney group ; and that, in 
every channel connecting the westerly and easterly seas that 
bound the coasts of this narrow cluster of islets, currents of tide 
propagated at successive intervals of time, would be naturally 
opposed to each other. The phenomena, then, which take 
place, from this cause, in the Pentland Frith, and the modifica- 
tions that the currents undergo from the islands in the channel, 
and from the form of the coasts, constituted the object of Mr 
Low's inquiries ; and, as the result of them has not, to my 
knowledge, been ever given to the public, the following account 
is selected from an unpublished Tour which he made through 
