How to obtain tt. 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 ex- 
perience 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 hatch- 
ery which I visited, I found that as soon as the eggs would bear 
