PORT SERVICES AS REGARDS SHIPPING 45
and the focal plane, or plane of illumination, is much
higher and is, therefore, effective for a correspondingly
greater distance. Lightships, which formerly necessitated
crews, can now be left unattended, by reason of the
modern automatic arrangement of the lighting apparatus.
Lighthouses, certainly those of the larger type, fall
outside the scope of these notes, as being coast signals
rather than adjuncts to ports ; though, as mentioned above,
certain lighthouses on the Welsh coast are controlled by
the Mersey Docks and Harbour Board. A lighthouse
being steadier than the floating structure, it is possible
to introduce important refinements and more intricate
mechanism, and to produce a light of a much more
powerful and important character.
Dredging. The dredging and maintenance of the fairway
 and all operations for the improvement of the channels
 come within the purview of the harbour authority.
This body may be identical with the port authority, as
it certainly is when the port is under autonomous management.
 But there are cases of dual control, particularly
in regard to railway ports. Taking Southampton as an
instance, the local Harbour Board is responsible for the
condition of the approaches to the port, while the entire
dock system, and, in fact, all the river quays, except an
unimportant town quay, are administered by the Southern
Railway. The two bodies, however, wisely co-operate
very closely, and such a policy is undoubtedly to the joint
interest of the railway and the town. The Railway
Company is represented on the Harbour Board and contributes
 liberally to the expenditure incurred in connection
with the waterways connecting the port with the sea.
The operation of dredging being entirely an engineering
function, it is unnecessary to enter into any details of the
process, but it is not inadvisable for a student of port
economics to understand something of the principles on
which it is based and in broad outline the methods by
which it is carried out.