The H timber and the Nile 81 



which a few bricks have been variously arranged. He will; 

 then easily realise how local conditions, impediments, &c, . 

 may influence the direction of the water and will readily under- 

 stand that the tidal strength may vary greatly in different' 

 places. We have but dealt with the surface of the subject. 

 There is a great mass of detail, to some of which we may, 

 recur at a future time. It will be something, however, if we; 

 have succeeded in correcting the very common misapprehen- 

 sion that the tide is merely a surface movement under the. 

 direct attraction of the moon and have substituted for it the 

 conception of a vast mass of deep water which exercises a ; 

 propulsive force upon the shallower. 



THE HUMBER AND THE NILE. 



It may seem a long step from the Nile to the Humber. These 

 two rivers, have, however, one curious feature in common. 

 The waters of both are, under certain conditions, laden with 

 mud, and are systematically used by farmers who live on their 

 banks, not for the sake of their water, but for that of their 

 mud. The methods adopted are not very dissimilar. The 

 Yorkshire farmer cuts deep trenches through the river banks, 

 and, having guarded them by strong wooden gates — "cleughs," 

 as they are called — he lets the water in over his fields and 

 allows it to settle. Usually the cleared water flows out again 

 with the falling tide, but if wished it may be impounded. It ( 

 is said that a twelve inch glass filled with Ouse or Trent water 

 in good condition, will deposit an inch of yellowish mud; not 

 sand, but clay. Thus it is not difficult, by taking several tides, 

 to raise the level of a field by several inches. The process 

 is locally known as "warping," "warp" being the name for 

 river silt. The warp thus obtained grows magnificent crops, 

 not perhaps to compete with those of the black mud of the 

 Nile, but still very good indeed. 



There are, however, important differences between the 



