38 REPORT OF COMMISSIONER OF FISH AND FISHERIES. 



of undermining or washing out the ends of the dam, it is best not to 

 undertake to raise a great head in this way. With any bottom except 

 one of solid ledge there is always great danger, and to guard against 

 it when the dam is more than 2 feet high may be very troublesome. If 

 there is a scarcity of water, or if it is desirable, for aerating or other 

 purposes, to secure a considerable fall, it is better to construct the dam 

 at some distance above the hatching-house, on higher ground, where a 

 very low dam will suflflce to turn the water into a conduit which will 

 lead it into the hatching house at the desired height. 



A square conduit made of boards or planks, carefully jointed and 

 nailed, is in nearly all cases perfectly satisfactory, and for an ordinary 

 establishment a very small one will suffice. 



The volume of water that will flow through a pipe of a given form 

 depends upon its size and the inclination at which it is laid. A straight 

 cylindrical pipe, 1 inch in diameter, inclined 1 foot in 10, conveys about 

 11 gallons of water per minute. The same pipe, with an inclination 

 of 1 in 20, conveys 8 gallons per minute; with an inclination of 1 in 

 100, it conveys 3i gallons per minute; with an inclination of 1 in 1,000, 

 it conveys 1 gallon per minute. A 2-inch pipe conveys about 5^ times 

 as much water as an inch pipe; a 3-inch pipe nearly 15 times as much. 

 A 1-inch pipe, with an inclination of 1 in 1,000, conveys water enough 

 for hatching 25,000 eggs; with an inclination of 1 in 50, enough for 

 100,000 eggs ; with an inclination of 1 in 20, enough for nearly 200,000 

 eggs. A square conduit conveys one-quarter more water than a cylin- 

 drical pipe of the same diameter. If there are any angles or abrupt 

 bends in the pipe, its capacity will be considerably reduced. It should 

 be remembered that if the water completely fills the aqueduct it is 

 en'tirely shut out from contact with the air during its passage, whereas 

 if the pipe is larger than the water can fill, the remainder of the space 

 will be occupied by air, of which the water, rushing down the incline, 

 will absorb a considerable volume and be greatly improved. It is 

 therefore much better to make the conduit twice or thrice the size 

 demanded by the required volume of water. If the bottom and sides 

 are rough, so as to break up the water, so much the better, and the 

 wider the conduit is the more surface does the water present to the air. 



AERATION. 



The water which fishes breathe is but the medium for the conveyance 

 of air, which is the real vivifying agent, without which fish and eggs 

 will die, and with a scanty supply of which the proper development of 

 the growing embryo is impossible. Water readily absorbs nir when- 

 ever it comes in contact with it, and the more intimate and the longer 

 continued the contact the greater the volume it will absorb. The ample 

 aeration of the water to be used in the hatching-house has already been 

 mentioned as a desideratum of the first importance, and some of the 

 devices by which it is to be secured have been alluded to. 



Water from either a brook or a river that has been torn into froth by 



