232 ARTIFICIAL POND-BEDS. 



the width of the current you wish to flow through 

 your ponds inside the defending wall. This 

 opening at the upper end of the wall is to be so 

 framed that whether the state of the river is low 

 or high, the supply of water to the pond will be 

 neither injuriously diminished nor increased. The 

 drain-mouth, or opening in the wall is to be se- 

 cured by a strong iron grating, the bars of which 

 are to be half-an-inch apart. This grating will 

 prevent the accumulation in the ponds of anything 

 hurtful to them. 



The bed of the ponds must be dug up to the 

 depth of about ^we feet, and they must be nine 

 feet in width, and eighteen in length. Their 

 bottom must be lower by five feet than that of 

 their feeder. The bottom, however, must not be 

 quite flat, but graduated, rising from the end 

 furthest from the head of the current towards the 

 opening or drain-mouth. The necessary inclina- 

 tion can be given to the bottom of the pond, by 

 beginning with a layer of gravel one foot thick at 

 the furthest end, and finishing off towards the 

 mouth with a layer of gravel, eighteen inches in 

 depth. The bottom of the pond will thus become 

 an inclined plane. The ova are to be deposited 

 at the top of the gradient, where you have finished 

 off with a layer of eighteen inches of gravel, in 



