Haupt.] l^S [Jan. 18, 



Again, the President of the Institution o{ Civil Engineers of Ireland, 

 and Engineer of the Port of Dublin, T. Pursur Griffith, writes with refer- 

 ence to the alluvial harbor at Ostende, Belgium : 



" It is not necessary to enter into a detailed description of the successive 

 additions made to the jetties and sluicing reservoirs * * * suffice it to 

 say, that the jetties extend at present about 300 metres seaward from the 

 shore line, and the maximum sluicing capacity of the reservoir is about 

 1,100,000 cubic metres. The result of these costly works cannot be re- 

 garded as satisfactory. The channel is still shallow, while the bar a short 

 distance beyond the pierheads still forms a dangerous obstruction. Depth 

 of water at the entrance to a port is more needful during rough, wild weather 

 than in calm, and it is just at such times that sluicing operations similar to 

 those at Ostende fail.'" 



Speaking of the jetty system in general, he says : 



"The system so generally adopted in Continental ports, of parallel or 

 nearly parallel jetties, extending only to comparatively shallow depths, ap- 

 pears to be radically wrong in principle. Their tendency, generally, is to 

 act as groins, and make the sandy shore extend outwards until the sand 

 passes around the pierheads where the action of the sea heaps it up in the 

 form of a bar." 



It seems unnecessary further to multiply instances of the failure of the 

 principle of parallel jetties in tidal waters, and it is confidently believed 

 that the single-curved barrier placed upon the bar as an obstruction to 

 flood-wave and sand movement will be found satisfactorily to fulfill the 

 requirements of these problems. 



APPENDIX A. 



Extracts from a paper, by Cliarles Henry Davis, Lieut, U. S. N., entitled 

 "The Law of Deposit of the Flood : Its Dynamical Action and Office." 

 Printed in the Smithsonian Contributions to Knowledge, Vol. iii. Referred 

 to a Commission consisting of Prof. 8. Agassiz, Prof. A. Guyot and Prof. 

 Joseph Henry, and accepted December, 1851. 



" The views in the paper* were founded upon observations and exami- 

 nations of various parts of the alluvial coast of the United States, through 

 a series of years, and led to the discovery that the shape, extent and dis- 

 tribution of the loose material of which they are composed— quartzose 

 sand— were chiefly determined by the action of tides." * * * "It 



* The author here refers to a previous memoir on the same topic. 



