CHEAP FIXTURES FOR THE HATCHING OF SALMON. 951 



work. It is therefore desirable to adopt all practicable means of re-inforcing 

 it. If the site of the hatching-house commands a fall of five feet or more, 

 the thing is easily done. Either in the conduit, outside the house, or in 

 the hatching-troughs themselves, a series of miniature cascades may be 

 contrived. The broader and thinner the sheet of water, the more thoroughly 

 it is exposed to the air, and if, instead of allowing it to trickle down the 

 face of a perpendicular board, we carry it off so that it must fall free 

 through the air, as in Figure 1, both surfaces of the sheet are exposed 



Fig. 1. ^->^ 



and the effect doubled. When the circumstances permit, it is best to in- 

 troduce these in the conduit, which, as already suggested, may be made 

 wide and open for that purpose. If the aeration cannot be effected outside 

 the house there is still opportunity inside. Two long troughs may be placed 

 side by side, leveled carefully, and the water be received in one of them 

 and pour over into the other in a sheet the whole length of the trough, 

 which, of course, would be a very thin sheet, and very effective. In the 

 hatching-troughs themselves, also, there is an opportunity for aeration, 

 either by making short troughs with a fall from one to another, or by 

 inclining the troughs and creating falls at regular distances by partitions or 

 dams, each with its cascade, after the fashion already described. 



The only serious difficulty is encountered where the ground is very flat, 

 so that the requisite fall cannot be obtained. In this case the best that 

 can be done is to make a very large pool, several square rods at least, 

 outside the house, and make all the conduits as wide as possible, so that the 

 water shall flow in a wide and shallow stream. 



It will of course be borne in mind that the better the aeration the smaller 

 the volume required to do a given work; and on the other hand it is 

 equally true that the greater the volume the less aeration is necessary. 

 When so large a volume as six gallons per minute for every hundred 

 thousand eggs is at command a comparatively low degree of aeration will 

 answer. But so far as known the higher the degree of aeration the better 

 the result, without limit, other things being equal, and it is therefore 

 advised to make use of all the facilities existing for this purpose. 



