C>0 Mr Low on the Currents of Tide of the Pentland Frith, SfC. 
“ Much has been said concerning these wells, and different 
accounts have been given of them ; some telling us they are oc- 
casioned “ by some hiatus in the earth below others, “ that 
there may be some secret conveyances through the rugged rocks 
into caverns at the bottom, from which they may pass into some 
other places, where they rise again, and that even in the same 
firth,” &c. 
u I should think the plainest account of the matter is this : 
The tide setting from the westward through Pightland Firth, 
meets with little or no resistance till it arrives at Swona, where, 
striking on the southermost point, it acquires new vigour, and 
sets off for S, Ronaldsha, with vast force ; but, as it is held in 
on the Firth side by the main stream, and finds little or no resist- 
ance on the side towards the eddy of Swona, this must natural- 
ly cause a wheel in such a rapid current, which will immediate- 
ly begin at the sharp comer of the island, and continue to whirl, 
in a greater or lesser degree, (according to the time of the tide,) 
till it gets a good distance from the island, into calmer water, 
and loses the velocity it had acquired at the point it set off from. 
This first whirl will be succeeded by another; the second by a 
third, and so on, as long as the stream has sufficient force to 
form them ; and, accordingly, it is true, in fact, that this phe- 
nomenon does not appear in any degree, till the tide has acquir- 
ed some strength in the sound ; and, growing stronger as the 
tide becomes so, dies away some time before high water, when 
all is calm, till the turn of the tide, at which time the very same 
thing is repeated, and for the same reasons, at the other corner 
of the island. With the ebb the tide sets down upon Swona, 
from the S. E. ; and the strength of that part of the stream run- 
ning through the sounds of Flotta, &c. towards Hoy, and keep- 
ing between Swona and Barthhead, must occasion the very same 
thing to happen in the western eddy, as the flood did in the 
eastern ; and this we see to be exactly the case : for the whirl 
sets off immediately as the tide makes for Cantick Head, in the 
same manner as it did before for S. Ronaldsha. Annexed is a 
drawing, which may help to make what has been said better un- 
derstood. 
66 Explanation of the Scheme , Plate II. Fig . i. 
64 In Fig. 1. a, is the main tide of the Pightland Firth ; b, that 
edge of it that sets by the south side of Waes, and bears straight 
