. Astronomy for high schools and colleges . e is apparently caused by masses of vapor streamingup from that side of the nucleus nearest the sun, and grad-ually spreading around the comet on each side. The ENVELOPES OF COMETS. 301 form of a bow is not the real form of the envelopes, butonly the apparent one in which we see them projectedagainst the background of tlie sky. Their true form issimilar to that of a paraboloid of revolution, surroundingthe nucleus on all sides, except that turned from the is, therefore, a surface and not a line. Perliaps its formcan be best imagined by supposi
. Astronomy for high schools and colleges . e is apparently caused by masses of vapor streamingup from that side of the nucleus nearest the sun, and grad-ually spreading around the comet on each side. The ENVELOPES OF COMETS. 301 form of a bow is not the real form of the envelopes, butonly the apparent one in which we see them projectedagainst the background of tlie sky. Their true form issimilar to that of a paraboloid of revolution, surroundingthe nucleus on all sides, except that turned from the is, therefore, a surface and not a line. Perliaps its formcan be best imagined by supposing the sun to be directlyabove the comet, and a fountain, throwing a liquid hori-zontally on all sides, to be built upon that part of thecomet which is uppermost. Such a fountain would throwits water in the form of a sheet, falling on all sides of thecometic nucleus, but not touching it. Two or three vaporsurfaces of this kind are sometimes seen around the comet,the outer one enclosing each of the inner ones, but no twotouching each Fig. 103.—formation op envelopes. To give a clear conception of the formation and motion of theenvelopes, we present two figures. The first of these shows the ap-pearance of the envelopes in four successive stages of their course,and may be regarded as sections of the real umbrella-shaped sur-faces which they form. In all these figures, the sun is supposed tobe above the comet in the figure, and the tail of the comet to bedirected downward. In a the sheet of vapor has just begun torise. In h it is risen and expanded yet further. In c it has begunto move away and pass around the comet on all sides. Finally,in d this last motion has gone so far that the higher portionshave nearly disappeared, the larger part of the matter havingmoved a^ay toward the tail. Before the stage c is reached, asecond envelope will commonly begin to rise as at a, so that twoor three may be visible at the same time, enclosed within eachother. In the next figure the a
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