952 EEPOET OF COMMISSIONEE OF FISn AND FISHERIES. 



6.— FILTERING. 



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

 was customary to lay them on gravel, and under these circumstances it was 

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

 spring water deposits a very considerable sediment, which might accumu- 

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

 water and thereby smother and destroy them. When the eggs are deposited 

 on trays, however, even though their upper sides be covered with sediment, 

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

 the water beneath the tray, though of course the circulation of water 

 through the tray is not perfect. The trays, moreover, offer the best facilities 

 for cleansing the eggs as often as may be necessary, and establishments for 

 the hatching of eggs of the salmonidce do not commonly receive them 

 until they have arrived at the stage when they can be safely subjected to 

 whatever washing and disturbance may be desired. It is not, therefore, 

 deemed necessary to introduce any considerable devices for filtering water 

 which is naturally very pure, as are lake and spring water commonly when 

 not subject to intermixture with surface water during rains. There are, 

 however, so many cases in which it is necessary to use water subject to 

 constant or occasional turbidness that some directions for filtering are 

 indispensable. 



In the first place, let the water from the conductor be led into a deep 

 tank, which may be termed the "settling tank," where the coarser and 

 heavier dirt will sink to the bottom. This may as well be located outside 

 the hatching-house, and for a small establishment a hogshead sunk in the 

 ground will answer. From the settling tank the water should be led into 

 a filtering trough inside the house, as shown in Figure 2, which exhibits 



one out of many convenient arrangements. This trough may be just the 

 length of the head or distributing trough alongside which it lies, or may 

 be much shorter, four feet answering well where little work is demanded 

 of it. For depth and width 15 to 18 inches are convenient dimensions. 

 If the water is introduced near the middle of the filtering trough the 

 current may be subdivided, part going to the right and part to the left, 



