PORT ECONOMICS 
(Queenstown or Cobhe) and elsewhere. River estuaries, 
when sufficiently enclosed, also provide very efficient 
harbourage, as is the case at Liverpool, where the mouth 
of the Mersey has an internal enlargement. New York is 
another instance of a commodious river mouth harbour. 
Artificial Harbours. In the absence of natural features, 
protection for harbours has to be sought by artificial 
means. Ports which found themselves sufficiently well 
protected in the days of small ships and local sea trade, 
have in many cases had to enlarge their accommodation 
by outlying extensions in order to meet modern develop- 
ments, and, in so doing, have been under the necessity of 
providing massive protective works against the onslaught 
of tempestuous seas. Thus, at Genoa and Marseilles, there 
are coastal indentations which form a nucleus of natural 
harbourage, and certainly provide shelter, but only to a 
moderate extent. The growth of these ports has exceeded 
their natural limits, and has been extended along the 
open coast-line, fronting which it has been necessary to 
construct massive breakwaters of masonry and concrete. 
Naples is another instance where a huge system of frontal 
breakwaters has been similarly developed. 
In a number of cases, ports have originated on un- 
protected shores, and have had to be provided with 
harbours which are entirely artificial, as at Madras, 
Sunderland and Zeebrugge. Generally, these take the 
form of one or two curving arms, springing out from the 
shore, with an opening in the latter case between their 
outer pierheads or extremities. The area of water thus 
enclosed affords a degree of shelter which is more or less 
effective, according to the direction of the wind, the width 
and arrangement of the entrance. and other factors of the 
case. 
Failing a proper harbour enclosure, some amount of 
protection is often obtainable in a Roadstead, which is a 
deep gut, or channel, approximately parallel with the 
shore line, and flanked on the seaward side by sandbanks