2 34 The National Geographic Magazine 



creasing their fall by regulating and 

 dredging their beds. The amount of 

 dredging annually by the government 

 and bj^ private parties reaches a very 

 considerable figure. Nearly all of the 

 sand and gravel used in dike-construc- 

 tion is obtained from the river bottoms 

 by dredging. 



THE DIKES A SYSTEM OF NATIONAL 

 DEFENSE 



In conclusion, a word should be said 

 about the practice of inundating lands 

 for defensive purposes. The efforts of 

 the Dutch to flood their country, as de- 

 scribed in Motley's Rise and Fall of the 

 Dutch Rcpitblic, and later again in 1672, 

 at the time of the war with France, are 

 well known to those familiar with the 

 history of the Netherlands. The meth- 

 ods employed in those days were not as 

 successful as they might have been, and 

 the blunders that were committed would 

 have led to disastrous results but for the 

 greater ignorance displayed by the at- 

 tacking party. Thus, in 1672, the 

 French army of invasion could not be 

 prevented from draining some of the 

 inundated lands, although their lack of 

 knowledge of the complicated situation 

 did not permit them to succeed at the 

 time. When cold weather set in the 

 manipulation of the water by the Dutch 

 was so defective that large areas were 

 allowed to freeze over, and the enemy 

 was actually enabled to execute move- 

 ments on the ice. 



The enormous strength of defensive 

 works of this class was, however, amply 

 proved and the Government at the pres- 

 ent day has provided an elaborate system 



for flooding, which forms part of the 

 militarj^ defenses of the lower provinces, 

 lyands to be flooded are provided with 

 special gage rods or bench-marks indi- 

 cating the depth of water required in 

 order to be effective. Special gates have 

 been constructed in the dikes where 

 water is to be turned onto the land, in 

 order to avoid the slow and undesirable 

 process of piercing dikes. The amount 

 of water that is to be drawn for such 

 purposes from bosoms and canals, the 

 discharge that is to pass the gates in a 

 given time, and the ultimate time re- 

 quired to flood a particular area to a 

 certain depth are quantities that have 

 been determined for each section of land 

 with a nicety which no one can fail to. 

 appreciate who is familiar with hydrau- 

 lic computations of the flow of water in 

 open channels and through orifices. 



There are at present about 1,000 miles 

 of sea dikes in the Netherlands. The 

 total length of dikes is difficult to esti- 

 mate, and even if it could be estimated 

 would mean but little, for it must be re- 

 membered that the dikes have for the 

 most part in the course of time been de- 

 stroyed and rebuilt repeatedly. It has 

 not been so much a question of building 

 them as it has been of maintaining them 

 and keeping them where they were. 

 Besides protecting the country from the 

 invasions of both fresh and salt waters, 

 the dikes have served to reclaim no less 

 than 210,000 acres, nearly all of which 

 are good, fertile land. It is to be hoped 

 that the stupendous project of reclaim- 

 ing the Zuider Zee will some day be car- 

 ried into effect, whereby there would 

 be added to the kingdom some half 

 million acres of land. 



