Page:Proceedings of the Royal Society of London Vol 2.djvu/108

Rh is described by Sir Everard Home. The ova are deposited upon its own shell; sometimes one only, sometimes several, contained in one chamber.

The animal found in the Argonaut shell by Mr. Cranch, had deposited eggs upon the lip of the shell; they were united by pedicles, like those of the Sepia octopus, and differed from those of the Helix ianthina, and other testaceous Vermes living in water, in having no camerated nidus, and in having a very large yolk to supply nourishment to the young animal when hatched; so that this animal, says Sir Everard Home, must be resolved into a species of Sepia; an animal which has no external shell, and which only uses the Argonaut when it occasionally gets possession of one.

Some naturalists not acquainted with comparative anatomy have thought they saw the Argonauta shell partly formed in these ova. The appearance they allude to is probably the unusually large yolk.

The construction of the heavens, in which the real place of every celestial object in space is to be determined, can only be delineated with precision when we have the situation of each heavenly body assigned in three dimensions, which, says the author, in the case of the visible universe, may be called longitude, latitude, and profundity. The angular positions of the stars given in astronomical catalogues, and on globes and maps, may enable us to find them by the eye or telescope; but their distance remains unknown; and unless a proper method for obtaining the profundity of objects can be found, their longitude and latitude will not enable us to assign their local arrangement in space. The method of parallaxes has succeeded with regard to objects comparatively near. The parallax of the fixed stars has also been an object of attention; and although the investigation has hitherto produced nothing satisfactory, it has given us a magnificent idea of the vast extent of the sidereal heavens, by showing that probably the whole diameter of the earth's orbit, at the distance of a star of the first magnitude, does not subtend an angle of more than a single second of a degree. To stars of a smaller size the parallactic method admits of no application.

Sir William Herschel proceeds to consider the local situations of the stars, and proposes a standard by which their relative arrangement may be examined ; that is, by comparing their distribution to a certain properly modified equality of scattering, in which it is not required either that the stars should be equidistant from each other, or that those of the same nominal magnitude should be equally distant from us. A certain equal portion of space is allotted to every star, so that we may thus calculate how many stars any given