2G0 



THE DEPTHS OF THE SE.L 



[chap. VI. 



there ought to be one or two tubs about two feet 

 in diameter and twenty inches deep, and each tub 

 should be provided with a set of sieves so arranged 

 that the lowest sieve fits freely within the bottom 

 of the tul), and the three succeeding sieves fit freely 

 within one another (Fig. 52). Each sieve is pro- 

 vided with a pair of iron handles through which 

 the hand can pass easily, and the handles of the 

 largest sieve are made long, so that the whole nest 

 can be lifted without stooping and putting the arms 



Fig. 52.— Set of Dredging Sieves. 



into the water. The upper smallest sieve is usually 

 deeper than the others ; it is made of a strong open 

 net of brass wire, the meshes a half inch to a side. 

 The second sieve is a good deal finer, the meshes 

 a quarter inch to a side. The third is finer still, 

 and the fourth so close as only to allow the passage 

 of mud or fine sand. The sieves are put into the 

 tub, and the tub filled up to the middle of the top 

 sieve with sea-water. The top sieve is then half 

 filled with the contents of the dredge, and the set 

 of sieves are gently moved uj. and down in the 



