Flow to obtain it. 139 
security to the works, and is one of the first and most important 
considerations in connection with a hatchery. 
For work on a small scale one of the most useful arrange- 
ments is shown by Fig. 12. There are many modifications, but 
the principle is the same. It may be worked against a wall as 
shown, or out in the open, the boxes being supported by a wooden 
frame. It may be inside a building or otherwise. One of the 
most important points to be observed in its construction is that 
the outlets are efficient. Be sure that they are capable of 
conveying all the water that will ever be required, and take care 
also that the screens are large enough. I have so often seen 
screens made by simply nailing a piece of perforated zinc on to 
the inlet end of a spout, that I feel it necessary to give this caution. 
A very simple way of making a screen is to nail three strips of 
wood, each of about six inches long, so as to form three sides of 
a square enclosing the outlet, and nail the perforated zinc on to 
them. I use these small hatching boxes occasionally for experi- 
menting, and in my own I prefer to have the screen the full width 
of the box, fitted as described for the larger boxes. The results 
are much more satisfactory, as full-sized screens do not so easily 
get clogged, and when the egg shells are about this is important. 
There are other reasons also, which will be referred to in another 
chapter. 
