86 HARBOUR CONSTRUCTION. 



Sir James Douglass states that the heaviest seas he has 

 experienced were on the west coast of England, about the Land's 

 End and the Scilly Islands. In the latter place a piece of rock 

 had been moved weighing 200 tons, and that not in the most 

 exposed situation. He would expect that in the most exposed 

 part of the coast a block of 300 tons might be disturbed by the 

 heaviest seas. 1 



During the construction of the Wolf Rock lighthouse, a 

 wrought-iron crane was erected on the tower for setting the 

 work. At the close of the working season (1866), the bare post 

 of this crane a ivrought-iron cylinder 16 inches in diameter, 

 and the metal of which was 1J inch in thickness was left in 

 position, standing 20 feet above the masonry and 23 feet above 

 high water of spring tides ; but during the winter it was broken 

 off flush with the surface of the work. When an examination 

 was made of the material at the fracture, it was found to be of 

 excellent quality. 2 



Almost every one who has been employed in the construction 

 of sea- works in exposed positions, could supplement what has 

 been said by stating his own experience ; and it is scarcely too 

 much to say that, however much may be written on the subject, 

 only those who have had actual experience of what the power 

 of a storm-wave is can adequately appreciate it. 



Mud as an Indication of Exposure. The late Mr. Thomas 

 Stevenson, in his work on the " Design and Construction of Har- 

 bours," gives some interesting facts in regard to mud as an indi- 

 cation of the measure of exposure of a site, and points out that 

 the level below low water at which mud reposes is proportionate 

 to the amount of exposure, and consequently to the greatest 

 height which waves are likely to attain in any given locality. 



While admitting the value of the presence of mud as an 

 indication of the extent of exposure, when taken in conjunction 

 ivith other physical conditions, I am scarcely prepared to go so 

 far as Mr. Stevenson in regarding it as "both a delicate and 

 certain test of the limits of the utmost possible extent down- 

 wards to which the disturbance originating at the surface has 

 reached." 3 



If we take a tub full of water, and place some mud in it, we 

 may set up a miniature storm by agitating the water ; and if 



1 Miii. Proc. lust. C.E., vol. xxxvii. p. 373. 2 Ibid., vol. xxx. p. 14. 



* " Stevenson on Harbours," p. 15. 



