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 majs 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 toM'er, there is left 89 feet, as the height to which the water 

 is raised. This is equivalent to a hj^drostatic pressure of about 2^ 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 e<iukalent to a jnvssure ofrerii 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 tiU 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 Reid, the principal keeper of Ban-ahead Lighthouse, the assistant keeper, 

 and all the inhabitants of the little island, were cyc-witnesses of this curious exhi- 

 bition of the force of the waves ; and Mr Reid 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 bmy it out of sight, and the run extended up to the grass line above 

 it, making aperjwndicu/ar rise of from 39 to 40 feet above the high water level. 

 On the incoming waves striking the stone, yve could see this monstrous mass 

 of upwards of forty tons Aveight 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 

 Avith 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. 



