116 



SEASON 1874-75. 



in October are turning the stream into a vegetable soup, or 

 the frosts of December chill the water till it freezes to the 

 screens in passing through, or the drought has diminished its 

 volume till there is barely sufficient to fill the conducting 

 spout. The principle of safety is the same : it is to be found 

 in the " theory of waste." To make this plain in the case of 

 water-supply (for the theory of waste applies to nearly every 

 process in fish-culture), we have only to remember a chain is no 

 stronger than its weakest link. No more can be done with water 

 in fish-hatching than can be done with the least supply running 

 through the pipes on the driest day of the hatching season. After 

 this quantity has been ascertained, all over may and should be 

 regarded as waste ; the hatching-house tap or sluice should be set 

 to take this, and no more. Above the hatching-house tap (which 

 in the case now under consideration is in the filter-box) an over- 



FIG. 63- scale fa. 



flow must be formed, care being taken to make it as wide as space 

 will allow. So long as a drop of water passes the overflow, so long 

 will the head of water remain constant, and (since the taps or 

 sluices are below the filter-screens, they are always free from 

 obstructions) the flow to the hatching-trays will be absolutely 

 constant. The theory of waste applies most strongly to the 

 intakes of the main supply ; but it may be convenient to consider 

 here the nature of the theory, more especially as to many it will 

 seem opposed to old notions. In Nature, " who never errs," young 

 fish are produced in quantities so great, that, if they all attained 

 the age of even one year, the waters would be overstocked and 

 pollute the riparian land. 



Where the conditions are so variable, and the limits between 



