42 Thomas Stevenson, Esq., on Harbour Screw-Cramps. 
Though the force to which marine buildings may be in some 
cases exposed is therefore very great, it must nevertheless be 
borne in mind that the surface of such works which is simul- 
taneously subjected to this force is very limited, and perhaps 
rarely embraces more than one or two courses of masonry. 
Every harbour pier of proper construction is of greatly more 
than sufficient strength, when viewed as a whole, to resist 
the shock of the assailing waves. The friction to be over- 
come in causing relative motion between even two or three 
courses, coupled with the resistance offered by the backing, is 
so great as, in most piers, to ensure their stability. Although 
when viewed as a whole, therefore, almost every pier is more 
than sufficient to resist being overturned en masse; and the 
friction and resistance of the backing, consequent on relative 
motion, is so great as to prevent disunion on the large scale ; 
yet there is hardly any building whose constituent blocks 
are in themselves weighty enough to resist successfully the 
dislocating action of the sea. Accordingly, if we inquire into 
the history of the failure of harbour works, we shall gene- 
rally find that those have occurred during their construction, 
and can be traced to the want of due protection while the 
works have been in progress. And thus it appears that blocks 
of stone which are insufficient of themselves, when first laid, 
to resist the sea, are rendered perfectly secure when kept 
down by the superincumbent weight of the courses which are 
afterwards built upon the top of them. The last stones of un- 
finished courses, also, though equally insecure while in such a 
state, are found, after the works are completed, and the 
courses “ closed in”’ at both ends, to resist for ages the great- 
est assaults of winter storms. 
Many instances may be adduced of harbours suffering 
great damage in the course of construction, which have, when 
finished, successfully withstood the efforts of after gales ; 
and of destruction to a whole work resulting from neglect 
in speedily repairing some trifling breach in the sea-walls. 
The north harbour of Peterhead, for example, during its con- 
struction in 1819, received damage from a winter gale, which 
(as estimated at the time) would require an additional expen- 
diture of about £3400 ; and this pier has, since it was finished, 
