If you examine the first cut, you will see small openings connecting all the cells with each other, and the continuous passage thus formed through the whole of the shell, is called the siphuncle. The inhabitant of the shell is an animal of the Sepia kind, but without the webs which serve the Paper Nautilus for sails, and with its arms not so long. It always resides in the outermost chamber, and it is supposed that it forms a new partition every year, so that the age of the animal may be known by counting the number of chambers. When the Pearly Nautilus wishes to rise to the surface, it pumps out the water from its shell through the siphuncle, and makes itself light. It floats often with its shell upwards, and at other times it moves along backwards for a considerable distance, by means of spouting out water over the front edge of the shell. It never floats with the same beautiful stateliness as the Argonaut. There is also the pretty little shell of the Nautilus Spirula, not much larger than a shilling, plate XVI, fig. 5, which does not contain a fish, but is merely annexed to the body of a Sepia, as is represented, fig. 6. Its use appears to be just the same as that of the shell of the Pearly Nautilus, to answer the purpose of a float. These two kinds are very rarely seen floating on the surface, though there must be many of both of them in the ocean, from the number of their empty shells which are found. Perhaps this arises from their not being so buoyant as the Argonauts, and hence they more readily dip under the surface at the approach of a vessel. It is now time for us to bid adieu to the sea and its wonderful inhabitants, since I have promised to tell you of some of the wonders of the sky, (though I remember a great many more things that I should like to tell you of). [Illustration] PART III. WONDERS OF THE SKY. CHAPTER I. COLOUR OF THE SKY. Before I tell you of some of the wonders of the sky, I should like to tell you what the sky is: it is true you know pretty nearly as much as I do about it, but you perhaps have not thought so much on what you know. The sky is everything above our heads which is not connected with the earth: thus, you may speak of a cloudy sky, or a clear sky; or of the stars, sun, and moon, being in the sky. It may however be more proper to restrict the word to the blue appearance which the atmosphere above us assumes when it is not concealed by clouds. This appearance is caused by looking into the air, which surrounds the globe to the height of forty or fifty miles above its surface, and not by anything which exists above it; for mountains, when seen from a great distance, appear of the same colour, evidently from the extent of the air between the spectator and them.