were incrusted with salt, "you must carry your philosophy a little
farther, Ralph. That water has evaporated so much that it is too
salt for anything to live in. You will require to add FRESH water
now and then, in order to keep it at the same degree of saltness as
the sea."

"Very true, Jack, that never struck me before," said I.

"And, now I think of it," continued Jack, "it seems to me that the
surest way of arranging your tank so as to get it to keep pure and
in good condition, will be to imitate the ocean in it. In fact
make it a miniature Pacific. I don't see how you can hope to
succeed unless you do that."

"Most true," said I, pondering what my companion said. "But I fear
that that will be very difficult."

"Not at all," cried Jack, rolling his towel up into a ball, and
throwing it into the face of Peterkin, who had been grinning and
winking at him during the last five minutes. "Not at all. Look
here. There is water of a certain saltness in the sea; well, fill
your tank with sea water, and keep it at that saltness by marking
the height at which the water stands on the sides. When it
evaporates a little, pour in FRESH water from the brook till it
comes up to the mark, and then it will be right, for the salt does
not evaporate with the water. Then, there's lots of sea-weed in
the sea; - well, go and get one or two bits of sea-weed, and put
them into your tank. Of course the weed must be alive, and growing
to little stones; or you can chip a bit off the rocks with the weed
sticking to it. Then, if you like, you can throw a little sand and