1104 KEPOET OF COlVrMISSIONEE OF FISH AND FISHERIES. [4] 



gree of temperature, aud also require very high and broad and conse- 

 quently expensive dikes. If ponds are constructed on strongly-sloping 

 ground, it may be recommended to construct several small and shallow 

 ponds, one above the other, in preference to one large pond which would 

 be very deep on one side. 



II.— CONSTEUCTION OF THE PONDS. 



• As a general rule it will only pay to construct ponds that need but 

 few earth- works. The best way is to use broad aud flat valleys, with a 

 gentle fall, whose sides are high enough to prevent an injurious over- 

 flow of the water into the surrounding fields, and which in some places 

 approach so near to each other that it needs only short dikes to con- 

 nect them. Such gentle valleys are found in nearly every part of the 

 country, unless it is absolutely level, and frequently they produce noth- 

 ing but a small quantity of sour grass. Frequently they extend with a 

 gentle fall for a considerable distance, so as to make it easy to construct 

 a number of x^onds one above the other, which is particularly advan- 

 tageous, because small and shallow ponds are, as a general rule, much 

 more productive than large and deep ponds. In that case the ponds 

 can easily be drained, beginning at the lowest one, and be filled again 

 from the highest one. (Plate I, Fig. 1.) 



In such valleys it will be easy to find the most suitable place for the 

 princij)al dike after, by simple leveling, the fall is ascertained and there- 

 by the size of the pond is determined. The pond is of course deepest 

 next to the dike, and gradually grows shallower towards the other side. 

 For most purposes it is best not to exceed an average depth of 2 meters ; 

 as a general rule ponds having an average depth of 0.50 to 1 meter are 

 the best; the outflow of the water must be carried though the dike, and 

 a ditch of sufficient breadth and suitable fall must be supplied for it. 



The dike may be constructed of different material. The simplest and 

 cheapest are earth-dikes which meet all requirements, and which can 

 be constructed without hiring foreign laborers. We shall therefore con- 

 fine ourselves to a description of such earth-dikes. Even in wood aud 

 stone dikes the main body of the dike is made of earth, which towards 

 the side of the pond is protected by a wall constructed of stones or ce- 

 ment or of beton. The principal use of such walls is to protect the dike 

 against the action of the waves in large ponds, whilst in small ponds 

 this protection may be obtained in a much simpler and cheaper man- 

 ner. 



The best material for constructing the dike is clayey soil which does 

 not contain much sand. Wherever such soil cannot be obtained, except 

 by going a considerable distance, the main body of the dike may be 

 constructed of gravel or some other material, and be made water-tight 

 by a layer of clay 20 to 30 centimeters thick, which is either placed on 

 the side of the dike toward the pond, or is made inside the dike whilst 



