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28 surrounding the nucleus on all sides except that turned from the Sun. Accordingly, nothing within the easy reach of an ordinary reader will bring the actual condition of things more clearly home to him than the simile of the onion, supplemented, as it may be, by a personal inspection of a simple jet of water rising straight up from the nozzle of a fountain, and presenting, when looked at from a near position, the outline of a curvilinear bell tent.

The formation of envelopes in the head of a comet, when such show themselves, will be easily understood by an attentive consideration of Fig. 17, the idea of which is due to Newcomb and Holden.

The diagram is intended to represent four successive stages



in the developement of the envelopes. The Sun is supposed to be above the diagram, and the tail below. When the appearance is as a, the cometary matter, whatever it may be, has just begun to start rising upon the nucleus. In b it has risen higher, and spread on each side wider. In c it has spread still further, and may be regarded as distinctly moving away from the nucleus but encompassing it on 2 sides though at a distance. Finally in d the movement and developement has proceeded so far, that the uppermost portions of the cometary matter has become so attenuated as often to have almost disappeared, the larger portion of the envelope having,