MANUAL OF FISH-CULTURE. 39 



dashing dowu a steep bed has absorbed all the air that is needed in LO 

 or 20 feet of hatching-trough, and demands no further attention on this 

 score; but if the water is taken from a lake, a spring, or a quiet brook 

 it contains less air, which may be so reduced before it gets through the 

 hatching house as to be unable to do its proper work. It is therefore 

 desirable to adopt all practicable means of reinforcing it. If the site 

 of the hatching-house commands a fall of 5 feet or more, the object 

 may be attained by contriving in the conduit outside the house, or in 

 the hatching-troughs themselves, a series of miniature cascades. 



The broader and thinner the sheet of water the more thoroughly it is 

 exposed to the air, and if, instead of being allowed to trickle down the 

 face of a perpendicular board, it is carried off' so that it must fall free 

 through the air, both surfaces of the sheet are exposed and the effect 

 doubled. If practicable, it is best to aerate in the conduit, which, as 

 already suggested, may be made wide and open for that purpose. 



If aeration can not be effected outside the house it may be done 

 inside by arranging two long troughs side by side, leveled carefully, 

 so that the water is received in one of them and poured over into the 

 other in a sheet the whole length of the trough. In the hatching- 

 troughs themselves there is an opportunity for aeration either by 

 making short troughs with a fall from one to another or by inclining the 

 troughs and creating falls at regular distances by partitions or dams, 

 each with its cascade, after the fashion already described. The only 

 serious difficulty is encountered where the ground is very flat, so that 

 the requisite fall can not be obtained, and in this case the best that 

 can be done is to make a very large pool, several square rods at least, 

 outside the house, and make all the conduits as wide as possible, so 

 that the water shall flow in a wide and shallow stream. 



It will of course be borne in mind that the better the aeration the 

 smaller the volume required to do a given work, and on the other hand 

 it is equally true that the greater the volume the less aeration is 

 necessary. When so large a volume as 6 gallons jjer minute for every 

 100,000 eggs is at command, a comj)aratively small amount of aeration 

 will answer. But, so far as known, the higher the degree of aeration 

 the better the result, without limit, other things being equal, and it is 

 therefore advised to make use of all the facilities existing for this 

 purpose. 



FILTERIN&. 

 Before the introduction of wire or glass trays for hatching fish eggs 

 it was customary to lay them on gravel, and it was then absolutely 

 necessary to filter all but the purest water. Even ordinary spring 

 water deposits a very considerable sediment, which might accumulate 

 upon the eggs to such an extent as to deprive them of a change of 

 water and smother and destroy fhem. When, however, eggs are depos- 

 ited on trays arranged for a circulation of water beneath, as well as over 

 them, as described below, even though their up])er side.^ are covered 

 with sediment, they are clean and bright underneath and remain in 



