PORTS AND HARBOURS 
or ridges, which, without necessarily reaching the sea 
surface, reduce the depth of water over them to such an 
extent as to act as an impediment to waves, causing them 
to break in passing. 
BREAKWATERS 
The principal feature of artificial harbours is the Break- 
water,! the object of which, as the name signifies, is to 
break up and disperse waves, and so prevent them from 
producing agitation of the water surface within the harbour 
area. Although the design and construction of break- 
waters is strictly a matter of engineering science, yet in 
view of the bearing which the constructional and main- 
tenance cost has upon the “ overhead ” or capital charges 
of a port, it is desirable to mention one or two governing 
factors in relation to their economical formation. 
Breakwaters are of two distinct types, with an inter- 
mediate or composite form. The two distinct types are 
the Rubble Mound and the Wall. The first consists of 
stone rubble or rough blocks of various sizes, deposited 
pell-mell in a continuous line along the sea floor and raised 
until the crest reaches above surface level, and the second 
consists of regular blocks of masonry or concrete, built up 
in courses, exactly as a wall, or of concrete deposited in 
mass within temporary moulds to the same general outline. 
The section of the former is roughly triangular, or 
trapezoidal, with a broad base, and of the second, 
rectangular with a narrow base. 
The relative advantages and disadvantages of each may 
be briefly summarized as follows— 
The mound can be constructed by means of unskilled 
labour, and is generally cheaper than the wall, despite 
the fact that its bulk is considerably greater. But the 
random blocks are easily displaced by rough seas, and 
2 Abroad, the word Mole is more commonly employed, especially 
on the continent of Europe ; and, in France, this is almost indis- 
tinguishable from another word, [etée, which, however, more closely 
resembles our English Jetty. But see Chap. II.