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 gravel into your tank, and the thing's complete."
"Nay, not quite," said Peterkin, who had been gravely attentive to this off-hand advice, "not quite; you must first make three little men to dive in it before it can be
