248 ECONOMIC GEOLOGY 
Estuary Works anD MoprLs—The conditions of estuaries 
are so complex that the effects of a change cannot always 
be foreseen. Models show the effects of projected improve- 
ments on tide and current, and have been often used since 
their introduction by L. F., Vernon Harcourt for the Seine 
(Proc. R. Soc., xlv, 1889, pp. 509-20) and the Mersey (ibid., 
xlvii, 1890, p. 142; Rivers and Canals, 2nd edition, 1896). 
The problems of coastal defence and harbour improvement 
are so varied that each case has to be judged by application 
of the principles of wind and current action to its special 
geographical circumstances, so that the scour may be ade- 
quate but not excessive, and may act where it is wanted. 
Any interference with the natural agencies may upset their 
balance and have disastrous consequencies unless all the 
possibilities have been carefully considered. 
1D. W. Johnston, Shore Processes and Shore Line Development, 1915 ; 
¥. Latham, Construction of Roads and Sea Defences, 1903; E. R. 
Matthews, Coast Erosion and Protection, 1918; Thos. Sheppard, Los: 
Towns of the Yorkshire Coast, 1012 : W. H. Wheeler. Sea Coast. 1602.