72 Flow to obtain tt. 
with it, and that, on refilling, some time must elapse before this 
stock of food can be reproduced. Especially is this the case 
when the pond is supplied by means of springs, or the compara- 
tively barren water of mountain or moorland streams. For this 
reason, a pond should never be let off unless for some very special 
purpose, as, for instance, when it may become needful to clean it 
out, or to change the stock of fish, or possibly to destroy pike or 
some other predacious fishes, which may have been introduced 
accidentally or otherwise. 
There are many ways of constructing a bottom outlet, the 
great need being to have one that will allow the water to escape 
without at the same time letting go the fish. This outlet should 
be made at, or rather below, the level of the deepest part of the 
pond. In the case of a large reservoir, where there is much 
pressure, the pipes by which the water escapes should be of iron, 
and should be very strong; but for a small pond, or one where 
the water is not very deep, good sound earthenware pipes will 
often answer the purpose. I would here point out the necessity 
that exists, in the construction of embankments and bottom 
outlets connected with ponds of any size, for the employment of 
a thoroughly competent engineer. So much often depends upon 
the strength of a bank, that too much caution cannot be observed 
in attempting to impound any considerable amount of water. 
I was once sent for to inspect a pond of some fifteen acres 
after it had been completed, with a view to advising as to the best 
course to pursue for getting up a good head of fish. I had never 
seen it before, and when I arrived I found it nearly full, and 
during my examination naturally asked a few questions as to the 
embankment, puddle trench, etc. This not being really part of 
my work, I did not say too much about it, but strongly advised 
caution, and the water was not allowed to rise to its highest level 
for some time, and then only very slowly. The bank was closely 
watched, and, as the filling went on, the water was discovered 
percolating through, and the owner at once ordered the bottom 
outlet to be opened. The outlet pipes were of earthenware and 
of considerable size, and as the foaming mass of water came 
pouring through them, it was observed that the pipes were 
breaking up and going away down stream. Fortunately the 
