fectly well. The time and quantity of these additions ought to be regulated by a hygrometer, the specific gravity of the sea-water being maintained at about 1027, which is the average density of the waters of the Atlantic. A tolerable approximation to accuracy, however, may be made, by marking on the vessel the surface-level at first, and always maintaining the same level. A glass cover greatly prevents loss from evaporation, as will be manifest by the condensed moisture on it, especially after a cold night.
Cleansing the sides.—Though a few Periwinkles, as already observed, will keep down the accumulation of green confervoid growth on the sides of the Aquarium, they will not do their work so regularly as to prevent an unsightly appearance. Hence, about once in a month, it will be well to take a stick with a bit of soft, clean rag tied on the end, and rub off the greenness. It is easily accomplished, as the adhesion to the glass is very slight; but care must be taken, not to disturb the animals or plants more than can be helped; and also not to remove any spawn that may have been laid on the sides of the vessel.
The accumulation of the green deposit, however, on the rocks and stones in the Tank, must not be cleared away, but be cordially welcomed. The spores of the Green Algæ, thus profusely scattered, soon form, all over the bottom and on all projections, a tender growth, which gives off oxygen-bubbles in astonishing numbers, conducing immensely to the health of the animals. As soon as this begins to assume a woolly or downy appearance, the success of the Aquarium may be considered as no longer proble-
