How to obtain it. 179 



needful, it is well to pass the water through a gravel filtering bed. 

 This is easily made by digging out another hole, say four feet 

 square and three feet deep, and half filling it with coarse clean 

 gravel, free from sand. On the top of this put a layer of fine 

 gravel, about an inch to two inches deep, and on this a thin layer 

 of thoroughly clean sand. The water should enter this simple 

 filtering tank at the top, and be drawn from the bottom of the 

 mass of gravel by means of a pipe. This pipe should be 

 perforated, in order the more readily to take in the water, which, 

 of course, will rise to its own level, and may be drawn off just 

 below that height. Such a filter requires no attention while 

 hatching is going on, after once it is set working, unless the water 

 be very dirty indeed. In such a case, it may be desirable to have 

 two filters, and while the deposit is being taken from one, the 

 other goes on working. Should a filter require cleaning, and be 

 run dry for that purpose, the deposit is found to consist of a thin 

 layer of mud over-lying the sand. This mud may easily and 

 quickly be removed, a little more clean sand added, if needful, and 

 the whole is again in working order. It is, however, hardly 

 needful to have two filter beds, as the water can be so arranged 

 that it can be shut off the filter, and yet kept on through the ova 

 bed, while the filter is being cleaned. The whole cost of making 

 a couple of settling ponds and a filter such as I have described 

 need not exceed twenty shillings. Some fish culturists prefer to 

 reverse the action of the filter, that is, to make the water enter it 

 at the bottom and flow off at or just below the surface. They 

 work very well either way for sufficient length of time to hatch 

 fully eyed ova, but of the two systems I prefer the first. It is 

 much more easily cleaned should necessity require it, and on the 

 whole, is safer in its action than the other. 



In some American hatcheries wire grilles are used, and seem 

 to work very well, but, on the whole, the results are not so satis- 

 factory as those obtained on glass. The iron wire of which they 

 are usually made has to be well coated with varnish. My 

 experience of varnish is that from various causes it comes off and 

 leaves the metal exposed This necessitates great care in the 

 handling and use of these grilles, and at one large American 

 hatchery which I visited I found that as soon as the eggs would 



