What Are Jetties?

Tho Seientifie American gives a familiar description of the jetty systom whioh is to be applied in the improvements at the mouth of the Mississippi, as follows: The delta of the Miasissippi is formed of narrow strips of laad, mosily low lying banks, through whioh tlie river winds mitil it makes its exit to the guif by a nunaber of narrow passes. In Bome of these ehannels previons attompts have been made to deepen them by dredging, with but partial success, however, as a single ñood has beeu known to carry down suffloient sediment to till them to their original depth; aud the current, besides emptying iiito the open water at tho mouths, speedily lol't at that point bars of blue c?ay, surinountabio only by light draught sliips. The gist of Oapt. Eads' plan will now bo readily apprehended when it is regarded is sluiting the point of deposit of these barriera from the shoal water at the entrance of one pass out iuto tiie deep water where filling up by natural causes is impossible. By this means the river is now to be made to ent out and bcout its owu channel across the present bar. To do this, it is obvious that the banks of the pass must be extended so as to lead the stream far enough out; another soetion of conduit, as ifc were, must be added, and this is now to be formed by the submariue dykes or jettios. The material oí which these struetnres are to be composed is willow twigsbound in bundies, ternied by engiueers "fascines," eight or ten feet in length, and about as rnany inches in tiiameter. A largo mimber of fascines at a time will be lashed together to form rafts, tho first of which will be from seventy-üve to two thousand feet in width, the largest rafts being sunk in the deepost water. The rafts will next be towed to the proper point, therc loaded with stones and submerged, and tlms the work will continue, one raft bung sunk above auother until the surface is reached. Each line of rafts will be narrower than the ono bclow it, until the upper eourse will not bo more than ten feet wide. The two walls which will thus bo eonstructed will be prolongntions of the bauks, aud between theni will form a channel with sloping sides. In the c-ourse of the interstioes of twigs and stones will till with Baad and mud, so that evontually two solid Biltamariue leveis will be pïodnoed. Vcry little pile work, it h said, will be requireil, exoept, perhapa, at the head of Bottth Pass, wliicli is teO buldet al whu-li Ulo jetties are. to lic built, in order to próvido for tho proper regola tions of tho volume of water in the now otuinuol at vai-ions stagiw ol Ihe. river. Onpt. Eacls lias -..Ircady bogwli lii winveys, in wJiicli work, togother witli the making of the neco.ssary contracis km material, labor, etc., i will lie con lumtíd. Tin', llrstroft, it isexpcclcd, will Ijc un k by tho begiunhig oi üctober next.