How to obtain it. 127 
pure to do without. But this is a very rare exception. At or 
near the point where it enters the hatchery then, construct the 
filters. For pure spring water half-a-dozen or more flannel screens. 
will usually be found ample, and often three or four will do. It is. 
better to have too many than too few, for they play a very 
important part in the success of the work. For fifteen years I 
have worked with no other filter, and now that a much larger 
volume of water is required in the hatchery the same method is. 
essentially successful, except that the water is first passed through 
a couple of settling tanks, which are found very useful adjuncts. 
\ 
f 
fed 
Fig. 6. 
A simple filter shewn in Fig. 6 explains itself. It consists of 
a wooden box, six wooden frames with coarse flannel stretched 
on them, sliding into grooves at a moderate angle, an inlet and 
an outlet, and the whole charred inside. The size depends 
entirely upon the amount of work to be done and the state of the 
water. As an example, I may say that I have incubated 
successfully half a million ova in the water discharged through a 
set of four (occasionally increased to five) flannel screens, of about 
seventy square inches each. A double set of these (for 
convenience in cleaning), each in a separate box, is used, the whole 
water sometimes passing through one box, but, as a rule, both 
boxes working. 
The filter boxes I have at present in use are twenty-four 
inches by twenty-four inches, and two of these boxes now working 
will pass 200,000 gallons of water per day, or enough to incubate 
four millions of ova. Behind each filter are two settling tanks 
built of concrete. This applies to the main hatchery only, two: 
