Scientific Intelliyence — Hydrography. 379 



after a gale from the north-east, one stone was moved measuring 

 fifteen cubic feet, or about one ton in weight, and thrown on the 

 beach, after having been built into the wall ; and a stone containing 

 18 cubic feet was moved 30 feet from its place ; while the pierres 

 perdues or mound-stones were washed down to a slope of about 4 to 1. 



The following instance, which occurred at the landing slip of the 

 Calf Point, Isle of Man, affords a proof of the great force of the waves 

 even in the Irish Sea. During a gale from the north-west, a block 

 was lifted from its place in the wall and thrown landwards, which 

 measured 123^ cubic feet, equal to about 10 tons weight. 



In the German Ocean, we can refer to the Bell Rock Lighthouse,* 

 which, though 112 feet in height, is literally buried in foam and 

 spray to the very top, during ground swells, when there is no wind. 

 It is, therefore, a very important station for making such experi- 

 ments, because the rise of the spray may be regarded as a scale by 

 which the results of the Marine Dynamometer can be checked or 

 compared. 



In the published account of this work there occurs the following 

 statement : — On the 24th October 1819, the spray rose to the 

 height of about 105 feet above the rock. " It may, perhaps, 

 therefore," says the author, " be concluded, that the maximum force 

 of the sea at the Bell Bock 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 equi- 

 valent to a hydrostatic 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 equivalent to a 

 pressure of very nearly 3 tons per square foot. 



At the island called Barrahead, one of the Hebrides, a remark- 

 able 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. 



* At such a situation as the Bell Rock, a column of water or of air could be 

 conducted into the interior of the house, and might, in the one case, shew the 

 force of each wave as it sti-uck the building by the rise of the water column ; 

 or, in the other, by a pressure-gauge, shew the same result in atmospheres by 

 compression. 



