DIKES AND THEIR EFFECTS. 63 



method of construction just described. It was doubtless this fact which induced the 

 German engineers to adopt and generalize in so remarkable a manner the use of sub- 

 merged spurs, the other advantages of which could not have been discovered except 

 by the experience acquired after their construction. In fact, in all that portion of 

 the Elbe which is under the control of Prussian engineers, and where they have 

 applied sills, the two banks to-day show a remarkable regularization, and one which 

 constantly improves, so that the indentations still visible between the successive dikes 

 are filling up and new and regular banks will before long be in existence. 



It will be seen from what has been said that regularization of the bed as well as 

 of the banks is considered necessary in Germany, and that they not only fix by a series 

 of dikes the width of the low-water bed but also secure this channel against scour by 

 means of transverse dikes. The result is that the bed rises to the level of these sills 

 and assumes a regular slope, just as the banks become regular by deposit between the 

 spur-dikes. 



The advantages of this system are summed up by M. Jacquet as follows: 



The nearly uniform distribution of the slope, and the consequent disappearance 

 of the bars over which the depth of water was not in harmony with the general regimen 

 of the river. 



The protection of the works of regularization, and in general of all the works that 

 were attacked by shore currents. 



The removal or transfer of the line of greatest depth and greatest velocity to a 

 certain distance in front of the shore works, and the consequent suppression of the 

 dangers which dikes might offer to descending navigation. 



The regularization of the depths and of the velocities in the same cross-section. 



The creation of a uniform depth of channel throughout the length of river subject 

 to the same regimen, and sometimes throughout the whole course of a river, as has 

 happened on the Elbe, so that boats can everywhere find nearly the same depth of 

 water. 



Materials. As a general thing cheaper materials are employed for dike construc- 

 tion than for works of canalization. To be satisfactory the dikes must concentrate 

 the water at all points where the river spreads out, and it is evident that, in order that 

 the entire cost of the works may not be out of proportion to the benefits expected, 

 the expense per lineal foot must be kept down. Nor is it necessary that the const ruc- 

 tion should be of great excellence, because dikes may be more or less permeable, 

 their chief purpose being that of giving depth to the river and direction to its current. 

 Frequently they are constructed of loose stone piled up in ridges, or of gravel protected 

 by a broken-stone covering, or of timber cribs filled with stone. That part of the 

 timber under low-water level may be considered fairly permanent, but the portion 

 above soon decays, and suffers from the attacks of ice and the current, and, if not 

 promptly repaired, the dike itself is endangered. Piles are also used, driven into 

 the earth and surrounded by stone, while mattresses of willows weighted with stone, 



