84 THE IMPROVEMENT OF RIVERS. 



plane were an ocean of the same depth as that thread or plane. As this thread separates 

 the parts of the 'levee the outside water fills up the split, and by thus preserving the 

 same height of water within the split as at the beginning of rupture, the levee becomes 

 completely rent asunder, and thus reduced in its aggregate power of resistance is 

 finally swept away. Porous materials, then, in water-banks, no matter what be their 

 weight in the banks, tend by the insinuation of water-threads between their parts to 

 the destruction of those banks this tendency, however, being greatest at the time 

 of the construction of the works, and least at the time when their adhesion shall have 

 been perfected by the coating of deposit over their external faces, and the insinuation 

 by filtration in their internal pores, of earthy matter. 



" Loam is much better for water-banks than sand. Thirty per cent heavier, it 

 meets all the conditions involved in leveeing on the ground of weight much better 

 than sand. Much stancher in its parts, it is superior to sand in all those serious objec. 

 tions applying to sand for the purposes of Water-tight embankments. The very best 

 of those soils obtainable under the present practice on the Mississippi for the purpose 

 of river banks is blue clay. Several kinds of this clay are found on the lines of the 

 levee works, but they are all subject to the disadvantage of a greater or less admixture 

 of fine sand. Perfectly impervious to water as they all are, the presence of sand lowers 

 their usefulness partly by involving a lighter weight, but mainly, and sometimes even 

 to a very serious extent, by giving them a tendency, especially after frosts, to melt or 

 run like marl in water. But notwithstanding these drawbacks the clays of the Missis- 

 sippi bottom furnish its very best material for leveeing." 



There is much in the class of material employed, without doubt, but there is also 

 need to place this material in a careful, proper manner. The best of materials will not 

 give satisfactory results if thrown carelessly into an embankment, while an inferior 

 grade of earth may be so built into a levee as to make a safe and permanent bank. 

 Sound earth is a good enough material for levees, but the bank must be carefully 

 built, of sufficient dimensions, and, especially with a light or treacherous subsoil, must 

 have its base extended by a banquette. 



The opinions of the Dutch engineers, who are probably the leading authorities 

 6f the world on the construction of levees, are worthy of attention in connection with 

 this subject:* "The earth of which the dike is to be composed must be such as to 

 cohere readily with itself and with the soil beneath it. The more cohesion the soil has, 

 the more it is to be preferred; and the more will its different parts unite and form a 

 compact mass which can oppose resistance to the water, and thus furnish a tighter 

 dike. Clay is thus the most suitable earth for dikes, and for the most part is to be 

 found along our coasts where dikes are to be built. It is to be procured, by prefer- 

 ence, from the outer side, but when the fore-shore is scanty or wanting, it must be 

 taken from the land side. Sand has very little coherency, and does not afford a water- 

 tight and strong dike. Peat and swamp soil have too little specific gravity, often less 



* Holland Dikes, Starling, Am. Soc. C E., vol. xxvi., p. 694. 



