28 MR THOMAS STEVENSON ON THE FORCE OF THE 
In the published account of this work there occurs the following statement :— 
On the 24th October 1819, the spray rose to the height of 105 feet above the rock. 
“It may, perhaps, therefore,” says the author, “be concluded, that the maximum 
force of the sea at the Bell Rock is to raise the sprays to the height of about 105 
feet above the surface of the rock ;” and deducting 16 feet, which is the height that 
the tide rises upon the tower, there is left 89 feet, as the height to which the water 
is raised. This is equivalent to a hydrostatic pressure of about 24} tons on the 
square foot. Since that time, however, there have been still greater proofs of the 
force of elevation. On the 20th November 1827, the spray rose 117 feet above 
the foundations or low water mark; and the tide on that day rose 11 feet upon the 
tower, leaving 106 feet as the height of elevation (exclusive of the trough of the sea), 
being equivalent to a pressure of very nearly 3 tons per square foot. 
At the island called Barrahead, one of the Hebrides, a remarkable example 
occurred during a storm in January 1836, in the movement of a block of stone, 
which, from measurements taken on the spot, is 9 feet x 8 feet x 7 feet = 504 
cubic feet, which, allowing 12 feet of this gneiss rock to the ton, will be about 42 
tons weight. This great mass was gradually moved 5 feet from the place where 
it lay, having been rocked to and fro by the waves till a piece broke off, which 
rolling down, and jamming itself between the moving mass and the shelving rock 
on which it rested, immediately stopped the oscillatory motion, and thus prevented 
the farther advance of the stone. 
Mr Retp, the principal keeper of Barrahead Lighthouse, the assistant keeper, 
and all the inhabitants of the little island, were eye-witnesses of this curious exhi- 
bition of the force of the waves ; and Mr Rerp also gives the following description 
of the manner in which they acted upon the stone. 
“‘ The sea,” he says, “ when I saw it striking the stone, would wholly im- 
merse or bury it out of sight, and the run extended up to the grass line above 
it, making a perpendicular rise of from 39 to 40 feet above the high water level. 
On the incoming waves striking the stone, we could see this monstrous mass 
of upwards of forty tons weight lean landwards, and the back run would uplift it 
again with a jerk, leaving it with very little water about it, when the next incom- 
ing wave made it recline again. We did not credit the former inhabitants of the 
island, who remarked that the sea would reach the storehouse which we were 
building; and when these stones were said to have been moved it was treated 
with no credit, and was declared by all the workmen at the lighthouse works 
to be impossible ; yet the natives affirmed it to be so, and said if we were long 
here we might yet see it. They seemed to feel a kind of triumph when they called 
me to see it on the day of this great storm.” 
Having now detailed the various observations and facts of which I was pos- 
sessed in relation to this subject, it may be necessary, in conclusion, to consider 
the general bearing of such an inquiry. 
